Remove inactive volumes to use the disk. Disk #3.

---

chopin70 wrote:

I removed all disks, factory reset, boot without disks in new GUI, setup password

That was a waste of time.  The NAS boots from the disks, and a factory reset formats the disks and does a fresh install of the system.  The volume configuration is saved on the OS partition on the disks.

So to start over, you'd need to do a factory reset the disks installed.  Then switch to flexraid and destroy the volume (which you can do while it is syncing).

On the original issue, the NAS simply isn't designed to handle the remove disks/reinsert disks sequence you've been using.  While you obviously did find a path that worked, you've been playing in the cracks of the piano. A better path would be to format a USB drive as btrfs, and enable checksums on that drive using ssh.  Then use backup jobs to copy the data.  Or just don't worry about the checksums.

I'd also suggest re-considering the use of volume encryption.  It clearly factors into the problems you are having.  IMO, the security benefit is small - it only defends against the threat of someone physically stealing the NAS (or just the disks) w/o stealing the key.  If someone has the entire system (including the key), they can do an OS reinstall to reset the admin password, and access all the files through the network.  

Alternatives to volume encryption include setting up veracrypt or vhd/vhdx containers on the NAS. These IMO offer stronger security, since they also defend against network access threats.   The main downside is that you'd only be able to access the encrypted files from one machine at a time.