Getting hard disk model and number under Linux

Try command as follows for /dev/sda (SATA). You need to pass -i or -I option which request identification info directly from the drive, which is displayed in a new expanded format:
# hdparm -I /dev/sda
# hdparm -I /dev/sda | grep Serial
OR for IDE /dev/hda
# hdparm -i /dev/hda
OR
# hdparm -I /dev/hda