Classically, hdX referred to a PATA drive while sdX referred to a SCSI drive. Sata drives will show up as sdX.
Newer kernels now use one of the scsi kernel modules for PATA drives as well, so on a newer Linux installation, you may not see any hdX references at all. They weren't happy with some of the pata code and how well it was maintained.
USB drives use scsi code and show up as sdX devices. SCSI drivers are used to handle USB drives in MS Windows as well, since USB is a bus that is how USB drives are implemented, and coded internally, pretending that they are scsi drives.
Last edited by jschiwal; 06-06-2008 at 02:20 AM.