Western Digital and Seagate are generally the best two brand names. The best type to note is those that see eSata capability over usb. But the 500gb WD usb model hasn't had one problem.
When first getting an external drive note that the factory partition will likely be Fat32 for compatibility with Mac as well as Windows. For Windows only a quick reformat to NTFS is advised. The drive will then be readily available while you may see the auto play popup appear if you don't download and unpack the replacement software from the support site.
Essentially the external drives are no different then those you install inside a case. The main difference is simply an external housing and external power source being an ac adapter. They do tend to run warmer however since there are no extra fans or HD coolers for them.
