Oh, don’t forget to activate TRIM after installing an SSD. If you don’t, it will get slower and slower over time. The reason is that OS X’s file system does not tell the disk to release space freed up by erased files until it wants to re-write in that space. That works fine with hard disks, but it slows down SSDs.

This is a good discussion of the issue:

https://www.howtogeek.com/222077/how-to-...

Or leave that slot empty to keep things simple if you don’t need the space. The 1.12 TB Fusion Drive was a way to get affordable SSD speed back in 2010-12. Nowadays, we can buy 500GB SSDs for less than £100 and 1TB ones for less than £200.

BTW: It doesn’t matter if you put your primary drive in the upper or lower position; you can boot from either. Oh, and just FYI: the “lower” position is the one closest to the bottom of the computer. You sometimes see this called the “upper” position because it is on top when the computer is upside down, as it is when you take it apart.

Have fun :-)

No doubt, putting all your system and application files on a SSD is the best and cheapest way to dramatically improve overall speed. Putting data on the SSD has less effect because most data aren’t that big or speed sensitive (photos and video excepted).

Two separate internal disks are not a natural fit with OS X. There are no A:, B:, C: etc. drive designators like in Windows, so the secondary, internal drive appears more like an external USB drive, except it works at full SATA speed, of course.

I have moved my music and all other user data except photos from my user folder to the secondary drive and use symbolic links to point to them, but if you use a large SSD as primary, that’s not necessary, and you may want to use the secondary drive in a different way.

TBC..

Apple’s Fusion Drive is indeed a “symbiotic” fusion between the SSD and the HD. Unlike “hybrid” drives, where the most used files are kept on the SSD in their ENTIRETY for speedy access, the Fusion Drive only keeps the most used PARTS of files on the SSD. In short, your files are spread over both disks in a way you can’t control.

So with the HD part of your Fusion Drive dead, you have to install new drive(s) and re-install your data from your backup. I did this and tried to re-install the Fusion Drive, which requires a special version of OS X, but failed several times and gave up in the end.

Instead I configured my Mac Mini with a 500 GB SSD primary drive and a 1 TB HD as a secondary, but of course you could put two separate up to 4 TB SSDs in the two slots, depending on your needs and how much you want to spend.

TBC…