Lately I have issues mounting and un-mounting USB disks.
The first time I insert a disk it mounts automatically in thunar when clicked but ..... after unmounting and removing it (the "safely remove" option doesn't work at all) it wants a (user) password to mount again.
Not sure where the issue is (in the tools scripts prolly) but it's a bit of a PITA.
Do others see this behaviour too or is just my machines?
This should be a permissions issue, probably related to sudo, so your /etc/sudoers.d directory was modified?
As always, a fast solution is to re-install in upgrade mode 
That is never a fast solution if you've customized a lot and ..... beside the point.
IMO it's not a sudo issue but related to the permissions that 'thunar' has. Not only mounting has issues but also unmounting.
Sometimes unmounting a disk makes X11/Enlightement crash resulting in a dark screen (backlight off).
Did you changed thunar preferences (Thunar Prereferences=>Advanced=>Disk Manager=>Stockage) ?
also do you use gvfs ? (it can also monitor disk devices and is part of gnome).
If so, it could be a conflict between thunar-volman and gvfs-udisk2-volume-monitor ???
Just throwing ideas 
