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Using external USB-C SSD as main drive?

blue2noise

macrumors regular
Original poster
Jul 23, 2008
118
14
Hello! My iMac 27” from 2014 is sadly dead and I am looking at a 2017 version that has USB-C ports.

is it possible to use an external USB SSD (ie Samsung T5) via USBC as the main boot drive and, more importantly, how will the performance be vs an internal SSD SATA? I’m pretty sure I t will be better than the included fusion drive.

Thanks for any feedback you can provide.
 

Zandros

macrumors regular
Sep 1, 2010
114
63
It is possible to boot from external drives. The Samsung T5 is not a very fast SSD (it's slower than my 2013 rMBP) and you might as well run it on one of the USB-A 3.1 Gen 1 (5 Gbit/s) ports (you get both an USB-C and an USB-A cable in the box).

Perceived performance is subjective and depends on what you will be doing with the computer. Maybe apps will take an extra bounce or two to launch, but how important is it to you?
 
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cruisin

macrumors 6502a
Apr 1, 2014
962
223
Canada
The 2017 should get about 2 GB per second read and 1.5 GB write for the 256 GB model and the write speed improving to about 2 GB with the higher capacity models. The T5 makes about 0.5 GB read and write so it really depends on what you see as performance. Surfing the web will be quick. For certain light tasks the fusion drive might even be faster. As a side note, newer computers have given up on SATA SSDs (mainly due to the slowness) and instead use PCIe.

Booting will be fine although you might need to hold "option" each time when booting to select the external.

See also: https://forums.macrumors.com/threads/external-vs-internal-ssd-speed.2090138/ and https://forums.macrumors.com/threads/2017-27-imac-ssd-speeds.2051983/
 
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Fishrrman

macrumors Core
Feb 20, 2009
20,431
7,246
Get an iMac with an INTERNAL SSD.
It will be 4-5x faster than any USBc SSD.

The only external SSD that will be able to "outrun it" is the Samsung X5 thunderbolt3 SSD (perhaps a few other tbolt3 drives).

If the iMac has a fusion drive, it will still be "faster than" using an external USBc SSD -- at least for a while.
 
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Aiwi

macrumors member
Oct 21, 2010
39
24
I’ve been booting from a T5 500GB for a while. Works fine! Read and write speeds are about 500MB/s, so just about what a regular SATA SSD would give you.

The mechanical relic of an HDD in the iMac itself will still spin up though, so it won’t be as quiet as a pure SSD solution.

After a while the HDD noise was the deciding factor for me to open the iMac and adding a proper NVMe SSD. I have to admit that the HDD is quiet though, but definitively not silent.
 
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mikehalloran

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Oct 14, 2018
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The only external SSD that will be able to "outrun it" is the Samsung X5 thunderbolt3 SSD (perhaps a few other tbolt3 drives).
Yes on the X5; no on other TB3 externals.

Dirty little secret:TB3 is limited to 4 lanes. In a twin-blade TB3 external, each slot is 3 x2. In a quad, each slot is 3 x1. You have to read the very fine print but it's there. A 970 EVO/X5 hits 2800–3100. A twin or quad RAID 0 may get up to the same speed using twice or 4x as many blades. If set up JBOD, you get full storage at 1/2 or 1/4 the speed.

Oh, you can't boot from a RAID 0 in Mojave.

I’ve been booting from a T5 500GB for a while. Works fine! Read and write speeds are about 500MB/s, so just about what a regular SATA SSD would give you...
Yes but it's 1/6 to 1/5 the speed of a fast NVMe 3 x4 blade like the 970 EVO.

What fun is that?
 
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CheesePuff

macrumors 65816
Sep 3, 2008
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803
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The 2017 should get about 2 GB per second read and 1.5 GB write for the 256 GB model and the write speed improving to about 2 GB with the higher capacity models. The T5 makes about 0.5 GB read and write so it really depends on what you see as performance. Surfing the web will be quick. For certain light tasks the fusion drive might even be faster. As a side note, newer computers have given up on SATA SSDs (mainly due to the slowness) and instead use PCIe.

Booting will be fine although you might need to hold "option" each time when booting to select the external.

See also: https://forums.macrumors.com/threads/external-vs-internal-ssd-speed.2090138/ and https://forums.macrumors.com/threads/2017-27-imac-ssd-speeds.2051983/

You just need to set your startup drive as the external once in System Preferences...
[doublepost=1564054419][/doublepost]I have a 2015 27" 5K and use a Samsung T5 as the boot drive on it, since I bought the machine used and it came with only a 1 TB Fusion Drive with a tiny 24 GB SSD. macOS and a few of the basic applications I use alone consumed it, so everything after that for the most part is placed on the HDD, slowing down access.

While the T5 is much slower then the built in flash drive (~500 MB/sec vs. ~1750 MB/sec), its much faster then accessing anything off the HDD (~90 MB/sec) so overall it ends up being better my use case.
 
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Trusteft

macrumors 6502a
Nov 5, 2014
652
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Back in 2002 waiting for the 7200rpm drives to be released (9.5mm), I was stuck with a 4200 or something drive for my gaming laptop. I ended up plugging in the USB 2.0 port an external drive. I can't remember if I installed just the games or the whole thing on the external drive, but I did see a huge improvement over the internal drive.

Since there are no technical reasons to stop you from using an external usb-c drive as a main drive, using an SSD as one I have no doubt you will see improvements in overall performance.
 
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nemoryoliver

macrumors member
Apr 9, 2013
90
22
Philippines
To the one's using the Samsung EVO 970 NVMe SSD on their iMac. Did you ever got any bootrom issue when you upgrade to newer Mac OS versions?
 
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drmeatball

macrumors member
Aug 3, 2019
73
54
Ellicott City, MD
Lots of confusing info in this thread.

USB-C SSD drives boot just fine (I do it on my 2018 mini, the internal storage is blank).

All things being mostly equal, the most you can expect from an SATA SSD in a USB-C enclosure is about 500MB/sec read/write. Limitations of the SATA standard. 3Gb/Sec. $20

All things being mostly equal, the most you can expect from an NVME SSD in a USB-C enclosure is about 1.2GB/sec read/write. 10 Gb/Sec. $40

If you absolutely need full speed, you want NVME SSD in a Thunderbolt enclosure. 40Gb/sec. $Expensive$

Even the slowest choice (SATA SSD) is lights years faster than HDD. Can’t go wrong with SSD.
 
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mikehalloran

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If you absolutely need full speed, you want NVME SSD in a Thunderbolt enclosure. 40Gb/sec. $Expensive$
There are things you don’t understand. 40GB/sec is not possible from any available external storage array currently being made for home use. Check the specs. 40GBs is theoretically possible with 80 SSDs in RAID 0. This also assumes that you could get that data down 4 lanes... and there’s the rub:

TB3 is limited to 4 lanes of data as I wrote earlier. An X5 or any RAID 0 array tops out around 3000mBs — same as the internal speed of a 2017–2019 iMac (or a 2015 upgraded to a 970 EVO). Some claim speeds of 3500mBs — one twin-blade claims up to 4000mBs RAID 0 but I’ve not seen real world speed that fast and no NVMe 3 blades are spec’d to put out 2000mBs to only 2 lanes. A quad-blade RAID 0 array tops out around 1500mBs and uses four 970 EVO blades to get there. JBOD increases capacity but a twin runs half speed and a quad runs 1/4 speed. Look it up. NVMe 4 blades require PCIe 4 motherboards — they run at NVMe 3 speeds otherwise.

Less expensive is to install the blade inside the iMac where it belongs. The price difference between an X5 and a 970 pays for the labor and puts the change in your pocket. $ave more by installing it yourself. Easy.

2015 and later — use a fast blade like a 970 EVO or a WD Blue 3D. Costs more than the ones below but a lot faster.

2013-2014 — an inexpensive, slow blade like a 660p or P1 is fine. The PCIe 3 x1 bus cannot take advantage of the 970 speed.
 
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nicho

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Feb 15, 2008
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There are things you don’t understand.

If you're going to start a reply with that sentence, at least have the decency not mix up your bits (b) and bytes (B) and milli (m) and mega (M) in your response to him. There's no such thing as a mB.

40GBs is theoretically possible with 80 SSDs in RAID 0 but if they were 2TB drives, your total storage would be 2TB.

Wrong. In RAID 0 your total storage would be 160TB. If 80 x 2 is too hard, give it a go here: https://www.gigacalculator.com/calculators/raid-calculator.php

It's an odd example to make because I can't imagine any raid controller capable of handling 80 disks in a single RAID 0 configuration effectively.
 
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nicho

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Feb 15, 2008
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Interesting way of not admitting that you were wrong. 40GBs external is not possible in anything available for home use.

You'll probably find that you were replying to someone else in the first place, so I wasn't wrong.

You'll probably also find that if you reread the post you replied to, nobody claimed such ridiculous speeds as 40GB/s. 40Gb/s was mentioned as the limit of TB3 - this is 1/8th of the speed of 40GB/s.

Assuming you're just incapable of understanding that there's a difference between bits and bytes, and you do in fact mean 40Gb/s - you're right in that no commercial product has come close to reaching this.

The X5 performs way above the limitations of USB-C 3.1 Gen 2 (or whatever it is that they call the 10Gb/s version of it) though. In comparison any USB-C drive would feel slow.
 
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mikehalloran

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You'll probably also find that if you reread the post you replied to, nobody claimed such ridiculous speeds as 40GB/s. 40Gb/s was mentioned as the limit of TB3 - this is 1/8th of the speed of 40GB/s.
Actually, somebody did but you’ll have to read the post I responded to to understand that. I quoted it.

I see that it wasn’t you — you’re just the one who criticized me.

There are companies within walking distance doing over 1TBs data transfer with controllers harnessing hundreds of drives. ok, its SCSI over wireless and you need the defense budget of a small country to afford it but it certainly exists.
 
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nicho

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Feb 15, 2008
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Actually, somebody did but you’ll have to read the post I responded to to understand that. I quoted it.

I read the quote, and I responded to it. They wrote a little b, not a big B. Thus, my comments to you.

not possible in anything available for home use.

There are companies within walking distance doing over 1TBs data transfer with controllers harnessing hundreds of drives. ok, its SCSI over wireless and you need the defense budget of a small country to afford it but it certainly exists.

You already outlined that we're talking about home use. Way to move the goal posts ;)
 
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nicho

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ondert

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Aug 11, 2017
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I wouldn’t get into that hassle. Get an iMac with internal ssd next time. Or.. bring your iMac to a reseller to change the internal hdd with a sata ssd.
 
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mikehalloran

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I wouldn’t get into that hassle. Get an iMac with internal ssd next time. Or.. bring your iMac to a reseller to change the internal hdd with a sata ssd.
Why in the world would you do that? A SATA III SSD is roughly 1/6 the speed of a fast NVMe 3 x4 blade in a 2015 or later iMac.

If you're going in — or having someone do the work — might as well do the job right.
 
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drmeatball

macrumors member
Aug 3, 2019
73
54
Ellicott City, MD
There are things you don’t understand. 40GB/sec is not possible from any available external storage array currently being made for home use. Check the specs. 40GBs is theoretically possible with 80 SSDs in RAID 0. This also assumes that you could get that data down 4 lanes... and there’s the rub:

TB3 is limited to 4 lanes of data as I wrote earlier. An X5 or any RAID 0 array tops out around 3000mBs — same as the internal speed of a 2017–2019 iMac (or a 2015 upgraded to a 970 EVO). Some claim speeds of 3500mBs — one twin-blade claims up to 4000mBs RAID 0 but I’ve not seen real world speed that fast and no NVMe 3 blades are spec’d to put out 2000mBs to only 2 lanes. A quad-blade RAID 0 array tops out around 1500mBs and uses four 970 EVO blades to get there. JBOD increases capacity but a twin runs half speed and a quad runs 1/4 speed. Look it up. NVMe 4 blades require PCIe 4 motherboards — they run at NVMe 3 speeds otherwise.

Less expensive is to install the blade inside the iMac where it belongs. The price difference between an X5 and a 970 pays for the labor and puts the change in your pocket. $ave more by installing it yourself. Easy.

2015 and later — use a fast blade like a 970 EVO or a WD Blue 3D. Costs more than the ones below but a lot faster.

2013-2014 — an inexpensive, slow blade like a 660p or P1 is fine. The PCIe 3 x1 bus cannot take advantage of the 970 speed.

I said Gb/s. Gigabits.
 
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drmeatball

macrumors member
Aug 3, 2019
73
54
Ellicott City, MD
FWIW, 40Gb/s is only about 5GB/s. I don’t think there are any consumer NVME drives that hit it yet but they are getting close (970 Pro does about 3.5GB iirc).

Anyway, great time to be computing :apple::apple::apple:
 
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