I am experiencing the same issue in the same scenery, modified my Ubuntu 12.04.1 LTS in order to start in console and I start my XFCE via startxfce4, the same problem mounting external drives/usb. Hope someone drop an answer for a fix.
Meanwhile, I deliver this workaround for future reference:
We can gain access to the unmounted usb/external drives by dropping the next command in a terminal.
sudo -S palimpsest
I know that we should use
gksudo instead of
sudo but if you do it that way you'll receive an error as in the next screenshot:
After providing our user password, the Disk Utility will be presented so that we can choose the drive/usb memory that we wish to mount and gain access to its files via file explorer (nautilus or any other) for read/write.
BUT! For that purpose we need to click in the link provided in the lower right corner with the label "Mount Point: Mounted at /media/DriveName"
This also works for my additional SATA Drives, which got compromised after the change that I made to my grub in order to boot in console (text) mode. No. This change isn't reverted by removing the changes I made to my GRUB.
Let's wait for an answer and meanwhile I hope this works for you the same as it does for me and may help future visitors.