I highly recommend purchasing an Epson viewer. The P-3000 (40 gb) , P-5000 (80 GB) and later models which are higher capacity hard drives, are excellent for backup. The other benefit is that there is a viewer, so you can see the photos on the 4 inch LCD with great visibility. I have had smaller backup drives as well, but without the viewer. With no viewer you don't know what you have, and they are not nearly as useful as being to view your photos on the device. With the viewer, you can show friends and family your photos without having to have a laptop or computer, or printing them all out. I have been travelling with my P-5000 for several years, and find it an excellent addition to my equipment. It comes with an AC adapter, but runs off rechargeable battery. With the proper energy converter for the country you are travelling to, you can use it where ever you go to charge it. It comes ready to accept SD and CompactFlash Memory cards, and since I also have Olympus XD, I purchased an additional USB connector for my Olympus cameras. I imagine you can purchase adaptors for the Sony memory stick, but I haven't checked that out myself. You can purchase various generic XD models that plug into the USB or the Compact Flash slot, or get a USB connector direct from Olympus. It comes with a Hi speed USB 2.0 interface to share with other USB devices. They are not inexpensive, but then when you want reliable quality for your photos, I believe it is worth the purchase price to have the viewer along with the back up drive.
Gerry