I have one of these iMac 5K which came with 128 GB Flash storage and 1 TB HD as Fusion Drive. Very quickly I replaced the disk with 2 TB SSD. For some time, I used the Flash as boot & apps & user folder, and kept big media (movies and Photos Library) on the SSD. Now I installed system and apps on the SSD as well, and use Flash only as a temporary storage.
But I though of one better setup:
Use the Flash as boot & apps, and use a folder on the SSD as my home folder (no hacking with symlinks, just via System Preferences as pictured).
I believe it could be fairly easy, but then I run into a roadblock:
What if I want the user volume encrypted?
Because, ideally, I would have the boot & app volume on the Flash unencrypted, and then make two volumes on the SSD - one encrypted for users' folders, and one unencrypted to store large media such as 4K media I'm occasionally editing (to avoid small but measurable encryption tax).
I have tried shortly with a Samsung T5 connected to my Mac:
created two volumes, Boot and Users
installed a system (10.15.3) on Boot
set up a user (skipping Apple ID / iCloud)
went to Users & Groups, unlocked, ctrl-click the user to get Advanced Options..., then selected a folder on that Users volume
rebooted, and it worked
then ctrl-clicked the Users HD to encrypt it using the same password as my login
logged out and in and it was OK
restarted and got You are unable to log in to the user account at this time. Failed because an error occurred
So it's a no-go but I wonder if either
a command such as
being logged in to iCloud, or
having the boot volume encrypted too with FileVault (as a last resort)
could help? I sort of understand it would have to be on an EFI or system level, and it gets more complicated with the fact Catalina has itself two volumes combining boot, and with more users on that encrypted volume (but FileVault volumes can be unlocked with more passwords?). Here's the desired layout:
Users HD [encrypted]
Final Cut Library
PS: there might be some other disadvantages of this setup, but here's one: when choosing Manage... in About they Mac > Storage, I get a message that "Some features of Optimised Storage will be unavailable based on your current configuration." I don't care that much as I usually use DaisyDisk to optimise the storage.