USB Hard Drive on Server

a SATA-2 drive directly on the system bus has much less overhead than a drive using a USB driver; and I don't believe that a USB drive can use DMA when processing the requests, so an internal drive will probably have a significantly faster access time than a USB drive.

Depending on your installation, there's also the possibility of a physical security issue with a USB drive - it could "just walk away" in the middle of the night perhaps?

Unless your server box is already crammed, why not go to a second drive, or a larger replacement for the current drive? You could use third-party drive management software to copy everything onto the new drive and preserve all of your installed software.

Reliability is USB Enclosures have a limited ability to cool the drive and where never designed for Full Time Use. The Drives overheat and fail.

Ideally if you need to use any form of USB Drive for any length of time it needs to be in a Vibrationally Suppressed and Actively Cooled Enclosure which are not cheap. The MX1 from Antec is an ideal case for this but even then I wouldn't be considering running a External Drive 100% of the Up Time of the Server.

http://www.antec.com/Believe_it/product.php?id=NzA0MjIx

You would be better off with some form of Network Attached Storage NAS Enclosure which is a different Kettle of Fish

http://en.wikipedia.org/wiki/Network-attached_storage

Col