Thanks for taking the time to report this! Sorry that PhotoStructure is misbehaving for you.
I believe I’ve addressed this in v1.0.0, but it’s not released as a stable build yet. Can you try an alpha build of v1.0.0?
If you still see this error on v1.0.0-alpha.7, can you DM me the result of running diskutil info -all and df -k -P on a terminal?
If you see anything you’d rather not share in that output, feel free to replace the text with XXX.