External Hard disc

There are two kind External HD - USB and ESATA.
There are two size of HD for external -- 3.5" and 2.5".
3.5" is a PC HD, 2.5" is a notebook HD.
The price of Segate 160G 2.5" HD is about 1650.
External HD plug into USB port then ok.

I myself have been using Seagate for more than 10 years, I am quite satisfy with its performance and durability. But it doesn't mean other brands are bad. Actually my company uses a lot of Hitachi 2.5" external HD, they are so far so good. don't have any problem.
Actually, most of the leading brands like Hitachi, IBM, Seagate, Western Digital, has almost the same quality of harddisk. All of them are good and durable. For years, I have been working with over hundreds units of different brand of HD (internal & external), all of them has equal chance of failure.
Brand selection is just a matter of choice.

What you should know before buying an external HD is:
1) What storage size you need? (20GB - 160GB)
2) What form factor? (2.5" or 3.5")
3) HD interface type (IDE, SATA, SCSI, SAS)
4) External HD case interface type (IDE, USB2, ESATA, IEEE1394, ....)
5) Cache size. (2M, 8M, 16M)

3.5" of course is much bulkier than 2.5", but its price is more cheaper than that of the 2.5".

Make sure the HD matches the HD Case in terms of interface type. Most of the external HD case comes with the USB2 interface. To avoid problem, buy them both from the same vendor and ask the shop if they are compatible with each others.