Sharing additional Time Capsule partitions

Interestingly enough, with the disk back in the Time Capsule, the second partition is being displayed along the regular "Data" in the Finder.

The info window correctly states the remaining free space (I copied some stuff onto it), that partition is accessed as a network volume using the afp protocol (format "AppleShare") and the Finder does not display the files I copied to that volume. This is also what df -h reveals:

//[USER]@[TCName]._afpovertcp._tcp.local/TM%20Backup%20Area  9.1Ti  2.3Ti  6.7Ti    26%  157668350   452666826   26%   /Volumes/TM Backup Area

//[USER]@[TCName]._afpovertcp._tcp.local/Data                5.5Ti  3.0Ti  2.5Ti    55%  798053666   666485672   54%   /Volumes/Data

I'm using MacOS 13.7 by the way. Now, How to: mount time capsule drive via the … - Apple Community

clearly states (and the output of df -h above confirms this) that the TC volumes can only be accessed directly via afp or smb. And smb is ruled out, because the TC ion course is not an SMB file server.

But the thread in the post I linked to in my first message strongly suggests that the OP could indeed access the files from outside. I just don't see how. Or how I could use mount_afp otherwise.

Now this is when I mounted the disk outside the TM, using a USB connection, the 10 TB partition being the additional one not to be used by the Time Capsule:

/dev/disk5 (external, physical):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        *16.0 TB    disk5

   1:                  Apple_HFS APconfig                1.1 GB     disk5s1

   2:                  Apple_HFS APswap                  1.1 GB     disk5s2

   3:                  Apple_HFS Data                    6.0 TB     disk5s3

   4:                  Apple_HFS TM Backup Area          10.0 TB    disk5s4

And this is the result when it's back in the Time Capsule:

/dev/disk5 (disk image):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      GUID_partition_scheme                        +15.2 TB    disk5

   1:                        EFI EFI                     209.7 MB   disk5s1

   2:                 Apple_APFS Container disk6         15.2 TB    disk5s2

/dev/disk6 (synthesized):

   #:                       TYPE NAME                    SIZE       IDENTIFIER

   0:      APFS Container Scheme -                      +15.2 TB    disk6

                                 Physical Store disk5s2

   1:                APFS Volume Backups von talisker... 3.2 TB     disk6s1

In the user manual of Sequoia it says that you can use (among others) an "AirPort Time Capsule" for backups with TimeMachine. So I reckon it would be safe to upgrade to Sequoia and still be able to backup to my old 4th gen. TC (model no. A1409).

Still, I didn't yet find any clue to answer my original question.

Because I want to use a part of the disk otherwise. 8 TB for the TimeMachine is enough for the foreseeable future, but, by default, TM claims the whole disk for itself.

That doesn't help me with my original issue, but you just successfully kept me from upgrading my Mac to Sonoma/Sequoia. This is no good news.