OK, this is starting to make more sense now 
Changing the path would lead to this behavior, and doing a gather state reset is certainly one way to address it.
It’s been a while since I’ve actually done a server migration, but I believe the manual modification required would be to update all the paths in the GatherState.XML (…\BES Server\Mirror Server\Inbox\GatherState.xml) to the new location (in your case, to the new path on the E drive). Worth testing of course to validate!