It's unfortunate that the problem didn't happen to reappear because that
sort of thing reinforces the bad old microsoft troubleshooting methods
(#1 - reboot, reboot, reboot or, if #1 doesn't work, #2 - reinstall).

Generally speaking, if something doesn't work in linux, you can find
what's misconfigured and fix it - rebooting generally doesn't work,
though it can be a lazy man's way of running the system startup commands
without having to do anything, or even to know what sort of things
happen on startup.

Too bad we didn't get to find out what your problem was. Sounds like you
tweaked something on that external disk, and in the process, cleared up
the permissions/ownership issues.