Setting up external SSD to replace Fusion Drive

[mich8261]

I came here looking for answers on how to “replace” the Fusion Drive in my late 2015 27” iMac but now I have decided to instead set up an external SSD as my start up drive And I will just ignore the old FD.

I am now looking for the best connection between the SSD and the Mac. The specs for my machine list USB 3 connectors and Thunderbolt 2. As I research SSD enclosures I am getting confused by all the variants (USB 3, USB 3.1 gen 1, USB 3.1 gen 2, USB 3.2 gen 1, USB 3.2 gen 2)

thank you

 

[rpmurray]

You can use any USB 3.x enclosure. The newer standards are backward compatible with the older, you just won't see the speed improvements. If you can find one at an affordable price go with the Thunderbolt 2 enclosures if you can, they smoke the USB 3 in that iMac. I use an OWC Thunderbay 4 with TB2. You can sometimes find a good bargain on them on eBay, especially if you get the Mini version which is designed for the SSDs. Make sure they are TB2 and not TB3 because OWC made both kinds. Although you should be able to use the TB3 version if you have the Apple TB3 to TB2 adapter.

 

[mich8261]

You can use any USB 3.x enclosure. The newer standards are backward compatible with the older, you just won't see the speed improvements. If you can find one at an affordable price go with the Thunderbolt 2 enclosures if you can, they smoke the USB 3 in that iMac. I use an OWC Thunderbay 4 with TB2. You can sometimes find a good bargain on them on eBay, especially if you get the Mini version which is designed for the SSDs. Make sure they are TB2 and not TB3 because OWC made both kinds. Although you should be able to use the TB3 version if you have the Apple TB3 to TB2 adapter.

That is helpful. Thank you

 

[Fishrrman]

A 2015 iMac has USB3, but doesn't have USBc (which could support USB3.1 gen2).

Your best bet is to get a USB3 SSD.
Either pre-assembled (Samsung t5 comes to mind), or... just buy a "bare drive" and an enclosure and assemble it yourself (really nothing to this).

Then erase the drive to one of the following:
- APFS, GUID partition format (for Mojave and later)
- Mac OS extended with journaling enabled, GUID partition format (for High Sierra and earlier).

Then, download either CarbonCopyCloner or SuperDuper.
BOTH are FREE to download and use for 30 days.
Doing things "my way" will cost you nothing.

Then, use CCC or SD to "clone" the contents of your internal fusion drive to the SSD.

Finally, go to the startup disk preference pane and set the external drive to be "the new boot drive". Then reboot.

I would recommend that you do not "ignore" the internal fusion drive, so long as it's still functional.

It can become "your new cloned backup" of the EXTERNAL boot SSD.
YOU SHOULD ALWAYS HAVE A SECOND BOOT SOURCE IMMEDIATELY AVAILABLE (shouting is intentional).

What are you going to do if the SSD fails?

 

[mich8261]

A 2015 iMac has USB3, but doesn't have USBc (which could support USB3.1 gen2).

Your best bet is to get a USB3 SSD.
Either pre-assembled (Samsung t5 comes to mind), or... just buy a "bare drive" and an enclosure and assemble it yourself (really nothing to this).

Then erase the drive to one of the following:
- APFS, GUID partition format (for Mojave and later)
- Mac OS extended with journaling enabled, GUID partition format (for High Sierra and earlier).

Then, download either CarbonCopyCloner or SuperDuper.
BOTH are FREE to download and use for 30 days.
Doing things "my way" will cost you nothing.

Then, use CCC or SD to "clone" the contents of your internal fusion drive to the SSD.

Finally, go to the startup disk preference pane and set the external drive to be "the new boot drive". Then reboot.

I would recommend that you do not "ignore" the internal fusion drive, so long as it's still functional.

It can become "your new cloned backup" of the EXTERNAL boot SSD.
YOU SHOULD ALWAYS HAVE A SECOND BOOT SOURCE IMMEDIATELY AVAILABLE (shouting is intentional).

What are you going to do if the SSD fails?

Thank you very much. I appreciate the detailed instructions.

 

[EugW]

Which Fusion drive do you have? If you have a 2 TB Fusion drive, that means you have a 128 GB SSD. If so, you can "un-fuse" it and run the drives separately.

 

[mich8261]

Which Fusion drive do you have? If you have a 2 TB Fusion drive, that means you have a 128 GB SSD. If so, you can "un-fuse" it and run the drives separately.

I have the 1TB version

 

[EugW]

I have the 1TB version

Unfortunately if you have a 1 TB Fusion drive I believe that means the SSD is only 24 or 32 GB.

 

[KenMDT]

Hey everyone - I JUST found this thread myself a few minutes ago. So I DO have a 3TB Fusion Drive running my Late 2014 27" iMac. I have a 1TB SSD External Drive I'd like to use as the primary and switch the Fusion Drive to just data. I have never heard anything about the "un-fuse"-ability of these drives. I'm totally open to either way, but I haven't been able to use this machine for Audio Production running Pro-Tools because the stupid Fusion Drive doesn't work! Any suggestions are greatly appreciated.

 

[EugW]

Hey everyone - I JUST found this thread myself a few minutes ago. So I DO have a 3TB Fusion Drive running my Late 2014 27" iMac. I have a 1TB SSD External Drive I'd like to use as the primary and switch the Fusion Drive to just data. I have never heard anything about the "un-fuse"-ability of these drives. I'm totally open to either way, but I haven't been able to use this machine for Audio Production running Pro-Tools because the stupid Fusion Drive doesn't work! Any suggestions are greatly appreciated.

Assuming your 1 TB external SSD is USB, I’d recommend unFusioning your internal drive (after making multiple backups of your important data), since your machine has an internal 128 GB SSD.

Use the 128 GB SSD as the macOS boot drive, and then the 3 TB hard drive and/or external USB SSD for data.

The problem with external USB SSDs is that most of them slow right down with sustained transfers. Your 128 GB internal SSD won’t have this problem.

 

[Fishrrman]

KenMDT:

EugW is giving you GOOD advice in reply 10 above.
You can "DE-fuse" the SSD portion and HDD portion of your fusion drive.

Now you have a 128gb internal SSD, and a 3tb internal HDD.

Make the SSD the boot drive, but keep it "lean and clean".
Put your large libraries on the 3tb HDD.

WARNING WARNING WARNING
You MUST back up the fusion drive BEFORE you attempt to De-fuse it!
This will DESTROY any data on the drive(s)!

When you get your "two-drive solution" up-and-running, then restore from your backup.

You will now have TWO internal drive icons on the desktop.
THIS IS NOT A PROBLEM -- you will quickly adapt to "what goes where".
(I keep FOUR internal drive icons -- partitions -- on my Mini at all times)

 

[Dmacmod]

I came here looking for answers on how to “replace” the Fusion Drive in my late 2015 27” iMac but now I have decided to instead set up an external SSD as my start up drive And I will just ignore the old FD.

I am now looking for the best connection between the SSD and the Mac. The specs for my machine list USB 3 connectors and Thunderbolt 2. As I research SSD enclosures I am getting confused by all the variants (USB 3, USB 3.1 gen 1, USB 3.1 gen 2, USB 3.2 gen 1, USB 3.2 gen 2)

thank you

Mich8261...Did you ever get you 27" iMac working with the external SSD? I use n Akitio Thunderbolt to PCIE case with an NVME as my boot up drive....I am getting around 1500 read and write.....its been working for about 2 years now...no issues..