Here are two commands that can help when dealing with physical Linux servers.

To get detailed information directly from the hard drive:

$ hdparm -I /dev/sda

To get a list of all the serial numbers for your hard drives:

$ for i in a b c d e f g h; do echo -n "/dev/sd$i: "; hdparm -I /dev/sd$i | awk '/Serial Number/ {print $3}'; done