Messed up encrypted APFS volume

[Taipan]

Hi!
I just made a mess. I got a new external DAS (Terramaster D4-320) for four disc drives connected to my Mac Mini. The four drives worked perfectly in another DAS before.
I don't know what I did, maybe the new DAS is crap, or maybe I accidently hot swapped the drives when troubleshooting some HD sleep issues.
Edit: In hindsight, I think what I did was: I hot swapped the position of the two drives in the enclosure, but prior to that I only ejected the volumes in Disk Utility, not the complete drives.

My problem now:
I have two drives with one encrypted APFS volume each, one is called "Time Machine", the other "Downloads". But now for some reason, BOTH of them show up as "Time Machine" in Disk Utility. And the one that should be called "Downloads" doesn't activate anymore. At startup, I get an error message saying "The volume 'Time Machine' cannot be unlocked. An error occured that prevents it from being unlocked" (roughly translated). It doesn't get to the point where I'm asked for the decryption key.
At some point, mac OS seems to have confused the drives and possibly wrote data to the Downloads drive, thinking it was the Time Machine drive.

I don't have high hopes for rescuing data from an encrypted volume, but maybe there's a chance that there's an expert here that can help me. There are about 4TB of data on the drive that I don't consider essential and are thus not backed up. Nevertheless I'd rather not lose them. :-S

I put the wrongly labelled drive back in to the old enclosure, which doesn't change anything, as expected.

Another thing that strikes me:
In Disk Utility, the wrongly labelled drive / Volume says "APFS-Volume - APFS (case sensitive, encrypted). I'm pretty sure the original "Downloads" volume was not formatted as case sensitive. I guess that mac OS uses case sensitive APFS for Time Machine, so this info seems to be taken over from the other drive as well.

Any help would be greatly appreciated!
Thank you!

 

[Taipan]

Some additional info:
diskutil shows the physical /dev/disk7 with a size of 8TB and /dev/disk7s2 as "Apple APFS container disk8" with a size of 8TB as well. This is correct.
Then for /dev/disk8 it shows an "APFS container scheme" with a size of 6TB. This is wrong, that's the size of the actual Time Machine container on the other disk.
Then it shows /dev/disk8s2 as an APFS volume "Time Machine" with a size of 3.1TB. I don't know where that number comes from.
There should be a single container and volume "Downloads" with a size of 8TB.

 

[DeltaMac]

Notice that your disk8 is not a physical disk, but is a synthesized (virtual) disk. And the Physical Store for disk 8 is on disk7.
disk7 is the physical disk, disk 8 is actually a synthesized disk. I think of it as a VM for the actual storage.(Don't know if that is an accurate use of "VM", but is simply how I look at that relationship with actual data storage
It adds to the confusion, when you only show the one physical disk, and the synthesized disk that would be software-associated with that physical disk. Only one is the physical disk.
(Might be better to show the full results of your diskutil list, as it may help you determine what is on each of the various types of disks, as displayed in that diskutil list.

 

[Taipan]

I'm aware of disk8 being a "virtual" representation of the physical storage container on disk7. That's why I posted these two, because together they represent the drive that I'm having problems with, and while the data for the physical drive seems to be correct, the data given for the "virtual" drive seems to be wrong. I don't understand why the other drives would be of any interest, unless you are referring to the "actual" Time Machine drive for comparison.
At the moment, I have unmounted the Time Machine drive out of fear to cause further "confusion", although that might be silly. But if it helps, I can mount it and provide the complete output.
Edit:
Here you go:

Disk 5/6 is the actual Time Machine drive, disk 9/10 is the one that should actually be Downloads. They look identical, with the exception of the physical size of the drive. I guess the partition data on the Downloads drive has been overwritten with the one from the Time Machine drive.
(I now realized where the 3.1TB comes from, apparently the size given is only the used space on the APFS volume).

Another thing I noticed: When the "false" Time Machine drive is connected at startup, I cannot mount the real Time Machine volume. I have to mount that one first.

 

[Taipan]

In case anyone is interested: I tried multiple recovery programs (Disk Drill, UFS Explorer, iBoySoft DataRecovery). UFS Explorer seemed the most promising, so after some fiddling, I contacted its support and described my problem. They were pretty friendly and said it would take manual inspection and "un-messing", and offered to take a look via Teamviewer, if I buy their software "Recover Explorer Professional" (which seems to be a somewhat trimmed down version of UFS Explorer). I wasn't quite comfortable with the thought, since there was some sensible data affected.
But I downloaded the demo version of the software and used it to scan the drive for lost APFS partitions. It did indeed find partition data of the Downloads partition and allowed me to scan it for files after entering the encryption key. After hours of scanning, it showed me all of the files in a "virtual file system" (including previews), which looked pretty complete. The demo version can only recover files with a limited size (I think 768kb or so), so I bought a license (182€) and a new drive, and I'm now in the process of recovering the files to the new drive. The process is pretty slow (the first 40GB took around half an hour via USB 3.0), but I'm happy that it's working. It looks like I'll get all the data back, and I guess the price is OK for not having a backup...