Make USB drive access more robust

This includes a couple of minor fixes to mitigate random crashes seen
in testing WeSay Send/Receive via USB drives.
I'm quite confident.  If the IsMounted value stays true, then MountPaths has to get set fairly soon.  If IsMounted changes to false, then we're out of the loop.
Underneath, this is referencing DBus managed values that are set by the system.