MacOS – Time Capsule showing monitor icon in Finder sidebar

I used Apple's Express Lane tech support. (Being as this was a brand new TimeCapsule, there was no fee.)

The tech told me that TC no longer supports backup to an external drive as of the 2nd Gen TC. He would not speculate as to why.

I can speculate that perhaps Apple wanted you to buy a bigger TC instead? Or perhaps (as was my experience) backing up to an external drive seemed to cause the TC to shut down (maybe overheated).

However, I can and did hook the drive directly to my USB port and I can back up that computer to it using Time Machine software (which is what I wanted to do anyway.)

Odd to answer my own question but this thread may provide an answer to the next guy.

Usually the sidebar shows Network Devices which are announcing themselves and are recognized by the 'Bonjour'-Service. For that to happen the appropiate Ports have to be used, in case of the 'Bonjour'-Service that would be Ports 1900, 5350, 5432 and 5353 (see "Well known TCP and UDP ports used by Apple software products")

My first suggestion would be to check your VPN-Server if these ports are blocked, which would prohibit the Devices showing up in the sidebar.

If the Raspberry is an SMB-Share you can try the following steps:

1. Open System Preferences » Network
2. Click the network interface you use for the shares (that probably should be your WiFi Connection rather than the VPN-Connection. You have to try both eventually)
3. Click on the Advanced button at lower right
4. Click on WINS tab, and manually enter the workgroup name of the server
5. Click the '+' under the WINS Servers list
6. Enter the IP of the server
7. Click OK, Apply, etc.