I have come across this before. While I do not have the full answer, this is a workaround I use
udisks2 or maybe it was before there.
I have purged whatever other mounting programs that I had for external pen-drives while making sure
mount package is there.
What I do is this every time :-
$udisksctl mount -b /dev/sdb1
Similarly when work is finished then :-
$udisksctl unmount -b /dev/sdb1
mount/unmount - the action that is desired. When you want to use the disk, use mount
-b = telling/sharing that it is a block device
/dev/sdb1 = this is where my usb disk is mounted. You can find where your usb disk gets mounted by running
dmesg | tail and then use the same path in the command.
A simpler way would be to use
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
sda 8:0 0 465.8G 0 disk
├─sda1 8:1 0 1.9G 0 part [SWAP]
├─sda2 8:2 0 93.1G 0 part /
└─sda3 8:3 0 111.8G 0 part /home
The issue/bug is known and reported. See https://bugs.debian.org/767233
Hope it is useful to you.
Update 23/09/15 - Try running /bin/mount and see if you still need the root password. Although mount does not require root password AFAIK.