when trying to access my external harddrive so as to back up my files and fix my WindowsXP i cant reach it, everytime i try to get to any of the USB ports... i get this warning, im betting this is a very common warning and i have spent hours upon hours of frustration trying to figure it out... and i still cant get it... the warning is as follows...

Could not mount device.

The reported error was:

mount: /dev/sda1 is not a valid block device.
any help would be immensly appreciated.