I had a similar problem with an older Sony Vaio laptop. Wanted to upgrade to a 20Gb from the installed 4.3. BIOS wouldn't let me do it. I tried flashing the BIOS (it wouldn't take), tried drive overlay software (wouldn't have that either), so I gave up.
The new 20Gb drive sat in drawer for several months until I decided to install it in a USB case and use it as a large "pen drive" for transporting my work from PC to PC. Lo and behold...the laptop has no problem seeing 20 Gb external!!!
Can't guarentee you'll have the same success, but suggest you ask around workmates / friends / works IT dept to see if you can borrow a drive overnight to try.