MacOS – The backup on Server is already in use

I think I have found the answer. The following site was very helpful:

http://pondini.org/TM/18.html

There are important differences between how Time Machine backups to local disks, and tonetworked disks. In the first case the backup goes to a "Backups.backupdb directory, in the second case to a "sparsebundle".

So my solution will be:

1. First have Timemachine start a new back up to my now networked Drobo.

2. Interrupt the backup after the sparsebundle has been made.

3. now move the Drobo temporarily back to my Mac, mount the sparse bundle, and use diskutil to erase it.

4. Copy the existing back up in to the monted sparsebundle.

5. Attach the Drobo again to the Time Capsule once this is finished.

However I can't try this out yet. First have to expand the disk space on my Drobo. So the first step for me really is: Get two 4TB disks on Monday...

No, you cannot. Backing up to a local volume (what you're having the server do) uses a slightly different backup format than a network backup (what your server is allowing clients to do). With a local volume backup, the backups are stored inside a folder named Backups.backupdb which is at the root of the backup volume (not in a subfolder). With a network backup, the backup is stored inside a disk image, and the Backups.backupdb folder is at the root of the volume inside that. Because the network backup is wrapped in a disk image, that image can be almost anywhere (volume root, folder, subfolder, whatever); but this flexibility does not exist for a local volume backup.