The repository resides only on the external HD, only congif information is kept locally. So, yes you would need to create a repository on each HD once. On the PC, when rotating HDs, you disconnect the repository, then connect to the repository in the othe HD. You can name the repository the same on both HDs (after all this is nothing but a directory on the HD).
Note also that Kopia offers repository synching kind of to achieve what you want. You backup (snapsot) to HD1, then regularly attach HD" and do:
kopia repository sync-to filesystem --path=path-to-repository-on-HD2
This will copy all new blobs from repository on HD1 to HD2
It will not delete any blobs on HD2, which have been deleted on HD1, due to kopia’s maintenance. If you want those to be deleted on HD2, too, add the delete options:
kopia repository sync-to filesystem --path=path-to-repository-on-HD2 --delete
Note that I have just donoe something similar, but wanted to rotate the HDs as you propose, so do snapshots to HD1 for a while and synch from HD1 to HD2, then rotate, i.e. snapshot to HD2 and synch from HD2 to HD1. This does not work. See Problem rotating external HD with “repository sync-to” on this forum.
Synching has the some disadvantages, though, over only rotating and snapshotting:
- If some file gets somehow corrupted, then snapshoted on HD1, the synch would copy the corrupte file’s blob over to HD2. If you simply rotate, there is the change of odentifying the problem and having a valid blob on the other HD.
- You need to connect both backup disk in parallel, so expose both backup copies to the remote possibility of some “damage”. If you simply rotate, you alway have a working copy, no matter what happens to the one attached to the PC.
Due the problem I have stumbled accross with rotating and synching (see above link), I’m thinking of going the rotate only way as you describe. Will have to do some tesing with this.