Disk corruption could very well explain this situation. As the disk labels sectors as corrupt, they are remapped to functioning ones, and the bad ones are discarded (a simplified version of the process).So if after 3 or 4 years of sitting in the wrong conditions, the disk may have decayed, leaving only 8mb of viable space left. Although this seems unlikely, it would explain your situation.
I would first try using this command from a command prompt
chkdsk /X /R dive_letter:
This should give you a basic idea of the total size of the drive that it is checking, as well as reporting any bad sectors it has come across. Should give you an output in bytes of the size of the drive. I had a problem once with a usb drive only displaying 1mb in the properties menu, but would function normally at its respective size--4gb I believe. Have you tried copying over a file larger than 8mb to see what happens? Just a shot in the dark, but you never know.
carnageX's idea will be your best bet if all else fails, Gparted is a great program which will display all partitions--used and unused space as well-- for the drive. It will display your hard disks as the default drive, but the option to change drives should be in the upper right hand corner. Will be something like /device/sdb for USB and /sda for your disk.