Quick and easy solution: Look for a /dev/sdb1 entry in etc/fstab and comment it out:
#/dev/sdb1 /media/usb0 auto rw,user,noauto 0 0
Source:Debian Forum. Explanation from dhunter in same thread:
This is exactly what happens if you install Debian from a USB drive, the installer adds sdb1 to fstab and whenever you plug it in the system will mount it as root. Always remember removing sdb1 from fstab after installing from USB.