I'm having the same issue with two of three usb flash drives. While one of them (a SanDisk flash drive) works normally, the system recognizes the other two flash drives (a Lexar and a Kingston) as being attached to usb ports, but it doesn't attempt to mount them for some reason.
However, I got them to load normally by restarting the computer and selecting the previous operating system kernel from the Grub menu during bootup. Ubuntu normally boots using the most recent kernel by default, but the previous kernels can be accessed and used to boot Ubuntu by selecting the Grub menu option "Advanced options for Ubuntu".
If your system is currently up to date, the current kernel in Ubuntu 13.04 (as of September 1, 2013) is 3.8.0-29 and the previous one is 3.8.0-27. If your grub menu is invisible during bootup because Ubuntu is the only operating system installed, pressing and holding the shift key during bootup should make it visible.
If the cause of your problem is exactly the same as mine, it appears to be related to the latest kernel update. I'm going to check to see if a related bug report has been filed at launchpad.
UPDATE: The latest Ubuntu 13.04 kernel 3.8.0-30 arrived today (Sept 6th) and it fixes this issue.