How to reuse HDD from TimeCapsule without losing the data?
This happens because the TimeCapsule disk contains 3 partitions with the same UUID (so much for being Universally Unique) equal to all &quot;0&quot;. macOS comes with a utility to assign a new random unique ID to the volume.
You need to attach the TimeCapsule drive to macOS and unmount all partitions (use diskutil list and diskutil info diskXsY to find the disks and ensure the zeroed-out UUIDs, unmount with diskutil umount diskXsY).
After that you can assign new UUIDs for the two 1 GB volumes, leaving the main data volume as-is (just in case).
/System/Library/Filesystems/hfs.fs/Contents/Resources/hfs.util -s disk2s1
/System/Library/Filesystems/hfs.fs/Contents/Resources/hfs.util -s disk2s2

This should be enough to get the data volume to show up.
I'm not sure if TimeCapsule expects that UUID to be all 0. If it does then this operation will prevent the drive from working in TimeCapsule, and you can't set the UUID manually using hfs.util. The following snippet is constructed by observing the changes hfs.util -s does to the disk header. This is as safe as open brain surgery, but as far as I can tell the hfs.util only writes 8 bytes to one location on disk, 1128 bytes from the start of the partition. This python script zeroes-out that location, hopefully returning the drive to original state:
with open(&quot;/dev/disk2s2&quot;, &quot;wb&quot;) as f:
    f.seek(0x468)
    f.write(bytes([0]*8))


All in all, this is very dangerous, make backups, at least of the header portion of the disk, but it still may corrupt data.
UPD:
Turns out Apple has open-sourced the hfs.util. I’m correct, this script does all that needs to be done, but the hardcoded offset is still a bad practice, since the FS header may be in a different place on the real disk.
If anyone seriously needs to zero-out the UUID on disk - you should modify the hfs.util itself: https://opensource.apple.com/source/hfs/hfs-183/hfs_util/
Or you can ping me in the comments, I’m willing to do it if anybody actually needs this.