Splitting Fusion Drive, couldn't unmount disk error

[psymac]

Hi all,

Attempting to split my 2TB iMac 2017 27" iMac Fusion Drive to use the SSD and HD separately. Have followed the instructions, but am getting "Unable to unmount drive" error. Do I have to boot from an external drive in order to do this? The instructions I'm using don't note this, so not sure, thinking now I can't unmount a drive that I boot from.

https://www.lifewire.com/split-fusion-drive-apart-2260166
[doublepost=1512661199][/doublepost]Ok, my bad, that Lifewire is not the best for instructions, the Macworld article is better, and does note I have to boot from Recovery mode as I suspected.

 

[Fishrrman]

"Do I have to boot from an external drive in order to do this?"

Yes.
I may be wrong, but I don't believe this can be done from the recovery partition -- because "the split" will wipe out the recovery partition along with everything else.

Either boot from an external (bootable) drive, or perhaps you could create a bootable USB flash drive and boot from that, then go to terminal to split the drive...

 

[psymac]

Actually it did work from Recovery mode, I assume its getting the OS from the internet. Still thinking however Apple didn't put the faster SSD blades in the 2TB Fusion Drive, as I'm getting 'only' 750/2300mb/s R/W speeds for the 128GB SSD.