Booting from external disk.

[dcmaccam]

I have a 2019 iMac with a 1TB Fusion drive which is running Catalina. However I am currently booting this from a Thunderbolt connected Samsung X5 SSD. The original drive is just sitting with Catalina on it.

No I was thinking of having a try at Big Sur. So I was thinking of disconnecting the X5, re-booting the machine from 1TB Fusion drive and installing Big Sur on this drive.

Now a couple of questions:-

After I install should I be able to re-attach the X5 drive and be able to reselect this as the boot disk? This would enable me to switch between either operating system.

Furthermore would I be able to re-install Catalina back on the Fusion drive from my TM backup? Any particular issues I might encounter?

 

[Fishrrman]

Yes, you can do that.

If for some reason you find you don't care for BS and want to return the internal to Catalina, you can do it this way:
1. Use either CarbonCopyCloner or SuperDuper (get the latest versions, these are free to download and use for 30 days).
2. Boot from the X5 drive
3. Open disk utility
4. IMPORTANT STEP: go to the view menu and choose "show all devices".
5. Select the fusion drive and then erase it to APFS, GUID partition format
6. Quit DU and open either CCC or SD and "clone over" the contents of the X5 to the fusion drive.

That should restore it to Catalina (from the X5).