Actually it turns out the problem was of a different nature!
During configuration I set up the Seafile client to use the external drive as its default location for syncing new libraries. Looked like a convenient option to me for my use case. However, I wasn’t aware that the Seafile client also stores important data in the .seafile-data directory on this default path…. So when my external drive was not attached the seafile client was actually missing a lot of the necessary files to operate.
I’ve changed the default path to an internal disk, copied over the existing data, and now the seafile client will start without problems when the external drive is not available.