This replacement "blue Dell hard drive caddy" is an excellent fit for the Dell Dimension series using the caddy-mount.  Without the hard-drive caddy it is impossible to mount the hard-disk. On the Dimension systems, Dell used this specialized caddy to mount the drives.  Other Dell systems use the rail-type mounts.  I ordered this hard-drive caddy thru Amazon and received immediate shipment - caddy fit perfectly and I'm up and running in no time.  Excellent service.