There are several reasons for this:
1. External Hard drives are generally slower than Internal Drives.
2. Many of them come preformatted with Fat32, so you are limited to files no larger then 4gig.
3. They are more likely to fail because they are normally subjected to a greater likelihood of accidental bumping or dropping.
4. Many external drive enclosures are not properly cooled for continuous usage and thus lead to premature failure.
5. You often have no idea what brand of hard drive is actually inside the enclosure.
When you plug it in and have it turned on does the computer make any beeps or show in the systray that a new device has been detected? Also open the device manager (Control Panel>System>Hardware>Device Manager) and check under disk drives to see if anything is there regarding your external drive.
Edited by Trinpim, 09 August 2009 - 11:02 PM.