Expanding Time Capsule's storage with added USB Hard Disks

Make sure you have enabled file sharing for the USB discs. You can do this by running AirPort Utility, selecting Disks, and then select File Sharing.

For the hard disc on the Mac Pro you will need to enable file sharing from the Mac Pro. On the Mac Pro go to System Preferences, then Sharing, then enable File Sharing. You can then select what files to share (which I would assume would be the entire disc you were referring to) and determine which users you want access (or you could just give everyone access which would be anyone who connects to your home network).

I am having the same issue and the first solution did not help me. Please help.

If what you imply by:

We can see the disks on the network but can only find an archiving configuration.

Is that from a single machine only a TC drive can be used at any one time for Time Machine backups, then that is true.

Time machine can only backup one machine's hard drive(s) (all the hard drives on that machine if more than one) to a single TC hard drive. Subject to any exclusions you have specified - either an entire hard drive/partition or folders to be excluded.

You can back up different machines to different TC hard drives.

For example on one machine with 2 hard drives, you cannot back up one hard drive to the TC disk and the other to one of the USB attached drives. Similarly you cannot backup some folders on a hard drive to one TC disk/partition and another to a different one, without a lot of manual intervention switching drives and forcing backups.

Nor can you seamlessly backup one machine such that after filling one TC hard drive it will continue on another drive. You can archive or switch drives when the one being used fills up. This allows you to search one and then the other in Time machine. But it does not present a seamless composite that spans the backups on 2 drives.