FileVault encrypting 1.5 TB in 1 minute!

[mschmitt]

I'm having a FileVault encryption anomaly, which is probably not due to SuperDuper! but I'm not able to figure out a Google search to reveal the explanation. I'm hoping someone has a clue.

I'm now cloning my 3rd external Catalina backup of 3. This is another 1.5 TB drive, just like the one I did 4 weeks ago.

My procedure was:

1. With (Mojave) drive connected to my iMac, use SuperDuper! to create the initial Catalina clone, by the erase-and-copy method.
2. Disconnect from the iMac and connect to my MacBook Pro.
3. Reboot the MBP in Safe Mode (to reduce number of processes running) off of the newly cloned external drive.
4. Turn on FileVault.
5. After FileVault encryption starts, disconnect from MBP and connect back to the iMac, as an external drive (not the boot drive).
6. Let the FileValue encryption process continue

The last step normally takes days. But what I'm seeing is that when I first unlock/mount the drive, it shows as 5% encrypted, but then after a few minutes changes to just "Yes (Unlocked)", as if it is fully encrypted! Same in fdesetup: it says "FileVault is On".

If I unmount it, it says encryption is 0%, paused. If I mount it again, the same thing happens: starts as 5%, then goes to what it would be if 100%.

It can't possibly be fully encrypted, since this has only been an hour or so.

Any idea what's going on?

Note that I've managed to encrypt 4 other Catalina drives: the MBP, iMac, a 4 TB SD, and the other 1.5 TB HDD. Only this last one has this anomaly.

Oh, and one more thing: I did rename the drive after I did the SuperDuper! clone, before booting from it. I renamed the system volume using Disk Utility so that it would rename the Data volume to match, since renaming in the Finder didn't do that. Not sure if that makes any difference. But maybe because of that, it isn't encrypting the Data volume?