iMac - External SSD as a boot drive?

Discussion in 'iMac' started by TomOSeven, May 23, 2019.

  1. TomOSeven macrumors 6502


    Jul 4, 2017
    Hi all,

    I have an iMac 5k from 2017 which I love, but it's the base model with a Fusion Drive.

    Is there an easy way to permanently use a USB-C SSD as a boot drive?

    Could I just do Time Machine Backup, connect a USB-C SSD, boot into recovery and install OSX anew based on the backup, onto the USB-C drive?

  2. tomscott1988 macrumors 6502a


    Apr 14, 2009
    Answered your own question :)

    Other way is to use something like Carbon Copy Cloner, or install a new copy of Mojave onto the SSD and use migration assistant to move your files over. All very user friendly. Not really much need to use time machine.

    Just set the external as the preferred boot option in System preferences and your good to go.
  3. ukp macrumors newbie

    Feb 9, 2007
    As above, yeah you can. I use a Samsung T5 SSD on one of my imacs and it exclusively uses the external drive for booting and general day to day use. My internal drive is now used for time machine.
  4. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    Bad idea. USB is too slow. You can’t enable TRIM. Other issues.

    A Samsung X5 is 6x faster over Thunderbolt 3. Boot from that and you’ll have the same performance as if you bought an SSD iMac.

    To use the internal HDD for Time Machine, you’ll have to untie if from the SSD and format it Extended GUID but certainly doable.

    There are a number of recent threads that ask this same question. I went into greater detail in those.
  5. tomscott1988 macrumors 6502a


    Apr 14, 2009
    TBH for 90% of people a USB SSD is ideal. The X5 is 4 times the price and for most people the performance day to day wont be noticeable, 3000mb/s r/ws is great but way overkill for most people.
    A sata 3 ssd over USB 3.1 gen 2 is a cheap way to make you Mac feel infinitely more responsive. In all honesty trim isn't the end of the world either. Myself and others have had these set up for years with minimal differences in performance.

    Recently I bought a Thunderbay 6 from OWC and have a 1TB Samsung Evo 970+ M2 NVME over TB installed which runs 3000mb/s r/ws day to day use it feels almost the same as the 3.1 1tb budget SanDisk ultra 1TB I had before. Boots are near identical, program launches are similar.

    Its only when you are exporting large video files or big transfers where it is noticeable and a bottle neck but really that more advanced.

    Otherwise 550mb/s is more than adequate for 4k video and any other day to day task. 1TB ssds can be had for £80 and a 3.1 usb c caddy £20 so £100 vs £420 for the X5.

    Or you could buy a T5 for £170 but they perform very similar to a build at home variant.
  6. Fishrrman, May 24, 2019
    Last edited: May 24, 2019

    Fishrrman macrumors P6


    Feb 20, 2009
    USB-c SSD will make a fine external boot drive. You should get reads of about 430mbps and writes in the 350-375mbps range.

    IF your fusion drive isn't "too full" yet, and IF all the contents will fit onto the SSD, the fastest way to accomplish this is:
    1. Connect new SSD and use Disk Utility to format it to Mac format (either HFS+ or APFS)
    2. Download CarbonCopyCloner from here:
    (CCC is FREE to download and use for 30 days)
    3. Launch CCC and clone the contents of the internal to the new SSD
    4. When done, go to the startup disk pref pane and choose the SSD to be the boot drive
    5. Reboot.
    6. Done.

    If you'd like more speed, you could get one of the these two solutions:

    First solution:
    Samsung X5 thunderbolt drive.
    VERY VERY fast (will far outperform ANY fusion drive), but these are considerably more $$$$.

    Second solution:
    Get a "bare" nvme blade drive (MUST be "nvme")
    Get a USB3.1 gen2 enclosure, such as this:
    Put the drive into enclosure and set it up (same as above). Just click the button to open and "drop the drive inside".
    This will give you read speeds up around the 850-900mbps range.
    Not as fast as thunderbolt, but almost twice the speed of USB3.1 gen1.
    And the cost is VERY reasonable for the additional speeds you'll get.
  7. LLA macrumors newbie

    May 23, 2019
    Los Angeles
    Hey, miehalloran, I was told by apple and OWC (where I bought an enclosure in order to use my SSD to run the new iMac, that the thunderbolt connection would not make for a faster set up because the SSD speed (or whatever drive one has) is what dictates speed. The connection cannot make things faster than the drive itself. Do you concur?
  8. jharvey71884 macrumors 6502

    May 3, 2011
    2015 iMac owner here. Been using USB 3 Samsung SSD boot driver for two years. Did the same thing for 2013 iMac. Don’t listen to anyone that says it is a bad idea. More than adequate over fusion drive.

    Used a zip tie or two and mounted to iMac base.
  9. nambuccaheadsau macrumors 68000


    Oct 19, 2007
    Nambucca Heads Australia
    LLA that is correct although the TB drive will boot much faster than USB3.
  10. SaSaSushi macrumors 601


    Aug 8, 2007
    Takamatsu, Japan
    I would not recommend booting from a USB3 drive on a 2017 or later iMac with TB3.

    The Shell Thunderbolt NVME enclosure can be purchased for $151 and coupled with the NVMe SSD of your choice. Performance apparently matches that of the Samsung X5, depending on the SSD, at a lower price.

    More products will continue to be released and prices will only continue to drop.
  11. smirk, Jun 14, 2019 at 9:34 AM
    Last edited: Jun 14, 2019 at 9:48 AM

    smirk macrumors 6502a


    Jul 18, 2002
    Orange County, CA
    It is correct that the connection won't make things faster than the drive itself, but what you may not be aware of is that there are two different types of SSD drives.

    SATA SSDs look like a mini hard drive and use the same connectors as hard drives. They are faster than mechanical hard drives, maybe 500 MB/second as opposed to ~100 MB/s for a mechanical. These are ballpark numbers, I'm not an expert on drive speeds.

    NVMe SSDs look like a really long memory stick, and basically plug directly into a PCIe slot. They are much faster than SATA SSDs, maybe 1,500 - 3,000 MB/s.

    There is also something called TRIM, which is a process that cleans up unused blocks on an SSD. macOS does not support TRIM over USB but does over Thunderbolt, which is why people are recommending the use of Thunderbolt for external drives instead of USB.

    There seems to be two schools of thought on the TRIM issue. @Fishrrman believes that TRIM functionality isn't critical and that using a less expensive USB drive works perfectly well. @mikehalloran believes that Thunderbolt should be used for a faster connection and TRIM support.

    You may find this article interesting/helpful:
  12. faraway52 macrumors newbie


    Mar 3, 2019
    I have two 2TB X5 connected via TB3 to my iMac 2017. It is very fast and more expensive than the iMac
    you should go with a nice SATA SSD connected via USB 3 adapter if you have no hard facts to spend all the X5 money.

    ps. Yes you will have no Trim but how much $$$$ are you going to save and what will Trim do for you and your way of using the iMac?
    pps. Just plug the USB drive in, format it and run CCC and you are done. It is that simple.

Share This Page

11 May 23, 2019