I have a three and a have year old compaq presario laptop  

I have a three and a have year old compaq presario laptop. I have triedto use an western digital 120 gb external hard drive for a back-up but Ikeep getting a message that says "file missing, unable to supporthardware." I ran a disk check on the laptop to correct any error thatmight have existed. It still didn't work. I bought a seagate modelexternal drive and received the same error message. I brought the unitinto to "Best Buy'" They suggested that maybe I needed "Service Pack 2"for the Windows XP but product information on the internet said thatthese particular external hardrives are also supported by my laptop'sService Pack 1. I am at a lost. Best buy's computer Geek squad does notknow what the problem is. The computer runs perfectly for all otherapplications. Do you have any ideas as to what the problem is. I reallyneed a small external hardrive to back up 25,000 pictures. Please help.Thank