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macrumors newbie
Original poster
Oct 12, 2017
So, I have a late 2015 5K iMac with a 1TB 5400RPM HDD and would really love to be able to boot from an SSD.
I have a few questions I can't find the answer for, even when searching the forum etc.

Will running my OS from an external SSD be better than the internal HDD I currently have?
If yes - is there a preferred SSD & Thunderbolt caddy you would recommend?​

I have seen a lot of people suggesting this situation should be avoided and you should just get an internal SSD. I am a little weary of doing that myself - it looks so much more complicated than the non beveled design of iMac. Has anyone had any experience doing this? Does changing a HDD yourself void your warranty?

Thanks for any help.


macrumors Penryn
Feb 20, 2009
Don't open the iMac, and don't bother with a Thunderbolt drive, either.

Get a USB3 SSD, plug it in, and make that your "external booter".
It will run as fast, or possibly faster than an external thunderbolt drive.

I'd recommend either a Sandisk Extreme or Samsung T3 as the drives to check out. There is now a Samsung T5 which is reported to be a bit faster, but is priced higher.

Sandisk Extreme would probably do "all that you need".

250 or 500gb size should be fine.
Put the OS, apps, and accounts on the SSD, BUT...

... leave the "large libraries" (movies, music, pictures) on the internal drive. They don't "need the speed".

If you decide to put a fresh copy of the OS onto the SSD, I recommend that you also get a USB flash drive of 16gb (or larger) and create a bootable USB installer. Things just seem to go better that way...
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macrumors 6502a
Sep 19, 2010
I agree totally with all of Fishrrman's comments. Same poster has given me very sage advice before on matters Mac. (thanks FM!)

But, personally using an external booter can be risky especially if you have kids in the house or you're in an office where the external boot drive might be removed(due to theft or whatever). Just me two cents!


macrumors 6502a
Dec 20, 2011
Winnipeg, Manitoba, Canada
What @Fishrrman has suggested will give you a very respectable speed bump that is simple to put in place and will not cost you a lot of $$$.

However, with this setup, you will not have maximized the total performance available for the Mac that you have purchased. If that is fine with you, then please do not read any further, but for me (keyword here is ME and MY thinking), I want to extract every ounce of performance that I can get from my Mac. It is not a matter of what my needs might be, it is looking forward and making my Mac more relevant for a longer period of time and to do that you need to maximize your options

Your iMac comes equipped with a PCIe connection for the SSD, this is faster than the older SATA interface.

if you read the following article (please see link below), at the bottom of the page you will see a graph representing the speed capabilities of several Mac’s. Using the suggestion as described by @Fishrrman, the best performance that you can expect to see would be equivalent to the 2012 iMac (SATA 3 versus USB 3 throughput is very similar), whereas you can see what your system is really capable of with the right hardware.

Personally I usually have no hesitation in taking apart a computer, but looking at the guides for your iMac would make me step back and think about it a little bit. Given that you are probably still covered by Apple Care, if you take this apart you will forfeit your warranty. So, if it was mine I would take it in to Apple and ask them to upgrade the unit with a PCIe SSD. If you decided to do it yourself you likely could not find a third party drive to use anyway.

But hey…..that is just me and the way I would do it. In my mind, I am already spending a lot of $$$ on this hardware and I therefore want it to be the best it can.

Good Luck…..I hope it helps.


macrumors newbie
Original poster
Oct 12, 2017
Thank you all for your replies, much appreciated.

I ordered an iMac from the refurb store but it arrived and it was faulty.

Found one on eBay with an SSD so going to go for that.
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