I have been finding this problem with many different makes/models of USB/Firewire external drives, including the Maxtor OneTouch series, and the WesternDigital MyBook series, and several IDE2USB conversion kits. The problem is consistent on PCs running Windows 2000/XP and on Mac running OSX(only tested on Intel versions). I would assume the problem to be consistent on other OSes also, but have not tested. So far, in every case encountered, I was able to "reattach" the drive with the following simple procedure.
1. Disconnect all cables, including power, from the external drives (or converter/adapter if using a kit). No need to shutdown the PC.
2. Wait for one minute. (If you are in a hurry, you can try it sooner, but I have found the 1 min. "circuit cooldown" to give me consistent results in all my tests. Both the drive AND the conversion circuit need the "cooldown" for best results.)
3. Reconnect cables to the drives, connecting the power first.
Your drive should be re-detected, and come back online.
Notes to chained firewire users:
1. Chain the drives together AFTER connecting power, but BEFORE connecting to the PC.
2. If your PC uses the 4-pin 1394 connector (many laptops), I recommend you not chain your drives. Results are inconsistent and unstable. Only chain your drives if ALL cables involved are standard 6-pin to 6-pin cables.
One instance I encountered involved a Mac with two MyBook drives chained and RAIDed together. The user was complaining that the drives were recognized by the OS, but several folders containing the previous days work were missing. Following this procedure corrected that problem as well, and restored all work.