problems mounting mass storage devices after booting into Windows XP

After shutting down Windows XP and booting into Ubuntu Jaunty 64-bit with latest upgrades, mass storage devices that previously worked fine refuse to mount as a standard user. As root, they can be forced to mount, but even with the -rw flag they won't mount in read/write, just read-only. Booting into XP again and unmounting them through safely-remove-hardware does not help. Rebooting Ubuntu does not help. Apparently at random, this may fix itself with one of the USB devices only to occur with another. This has happened with both a portable hard drive and an iPod.

I'm not a technical sort of person but can collect information if told how.