This is happening to me too. Dunno about USBs but for DVD/CD drives I got around it this way...
In the terminal type
This is my result of that command...
From that I could see that the mount points for my DVD drives were '/dev/scd0' and '/dev/scd1'
stone@ubuntu:~$ more /etc/fstab
# /etc/fstab: static file system information.
# <file system> <mount point> <type> <options> <dump> <pass>
proc /proc proc defaults 0 0
UUID=10b7e5ec-a013-4747-acb3-703b3570f1d1 / ext3 relatime,error
s=remount-ro 0 1
UUID=842d5f56-c9da-47c6-9477-1af12059e8cc /home ext3 relatime
UUID=05DE-C6ED /windows vfat utf8,umask=007,gid=46 0 1
UUID=8cbba75e-91c2-4735-ab08-36490b034932 none swap sw
/dev/scd0 /media/cdrom0 udf,iso9660 user,noauto,exec,utf8 0 0
/dev/scd1 /media/cdrom1 udf,iso9660 user,noauto,exec,utf8 0 0
/dev/fd0 /media/floppy0 auto rw,user,noauto,exec,utf8 0 0
stone@ubuntu:~$ sudo mount /dev/scd1
mount: block device /dev/scd1 is write-protected, mounting read-only
mount: /dev/scd1 already mounted or /media/cdrom1 busy
mount: according to mtab, /dev/scd1 is already mounted on /media/cdrom1
So I then was able to use the terminal to mount them...
sudo mount /dev/scd1