Support for external pictures location (removable drives)
Please describe the problem you are trying to solve
On a company computer, or a computer with a small SSD, it would be useful to store the pictures on a removable disk or a network storage. It's possible to do that with Shotwell as-is, but one needs to be super careful not to launch Shotwell without this storage attached, otherwise it will mark all images as missing, taking a substantial amount of time, reconnecting the images afterwards also takes time.
Possible Implementation
Shotwell should realize that the path is to a removable drive if the whole path is inaccessible, not just single files. It then should ask if a storage is missing and give the opportunity to mount this drive before doing any changes to the database.