Comment 3 for bug 1930312

For the record, this is not specific to Ubuntu or Ubuntu MATE at all -- I see this on Gentoo regularly, for example. It seems to be caused by a race condition in Caja and UDisks which didn't appear until UDisks started mounting volumes under user-specific subdirectories of `/media` -- so for example `user1` has volumes mounted under `/media/user1` and `user2` has volumes mounted under `/media/user2`. In ye olden days, `user1`'s UDisks-utilizing process (such as Caja) would mount the volume; `user2`'s instance of Caja would wait for the volume to get mounted by `user1`'s Caja, then open the shared mountpoint at `/media/[volume]`. Nowadays this doesn't work in any case, since `user1` puts in the request to mount the volume first and the volume is mounted under `user1`'s private mount directory -- `/media/user1/[volume]`. I'm not sure how to fix this yet, though. Maybe (but just a WAG) UDisks needs some kind of integration with the display manager or logind?