JorgeB

If you then re-copy with --sparse-always it will me made sparse again.

You have to restart the array in normal mode.

Stop array, unassign that disk, start array, but likely the emulated disk will still be unmountable, and the array will be unprotected until the disk is rebuilt.

Local GUI should work but there's no link detected on the NIC, so fix that first.

Correct, and if it asks for -L use it.

And it should work, you can try again and post new diags if there is problem, before rebooting.

Not sure what you are asking, Unraid is an OS, you don't need any other OS installed.

Ideally you do this before rebuilding on top, but check filesystem on disk1.

You can add (not replace) one of the 1TB devices, wait for the pool to balance, when done you can replace the 250GB with the other 1TB.

It's already been announced that this will change in the near future, once multiple unRAID arrays are supported.

Do you still want to upgrade using the 1TB devices?

Same as btrfs, it checksums the data but since array devices are single device filesystems it can detect corruption but it cannot fix it, you'd need to restore from a backup, but at least would know if there is corruption and which files need to be restored. Once it's supported by openzfs it will also be supported by Unraid (this is for pools only, not array) That's basically up to the user, most will be fine with xfs, if you care about any of the zfs/btrfs features like checksums, snapshots, send/receive use it.

I also asked for you to create a bug report, but in any case, devices are not being decrypted, stop the array, click on each array disk and cache and change filesystem from xfs to xfs-encrypted, then start array. Also note that you don't need to do a new config to add a new pool, zfs or other fs.

Please create a bug report and post the diagnostics.