My favorite method of destroying old hard drives is take them completely apart and put magnets on the platters and scratching them up.  I let my kids make stuff out of them.   I decided for fun this time.  I will show you each hard drive as I take it apart.  How do they differ.  I have two SAS drives that are server grade.  I have three SATA drives.   This will take some time to do.   So stay tuned.

SAS Seagate Cheetah 15K.7 300GB

SAS Seagate Savvio 10K.6 146GB

SATA Seagate Barracuda 7.2K.10 750GB

IDE Seagate Barracuda 7.2K.10 500GB

SATA Seagate Barracuda 7.2K.11 500GB

Platter size difference. 
Top: SAS Seagate Savvio 10K.6 146GB: slightly smaller than Cheetah
Middle: SAS Seagate Cheetah 15K.7 300GB
Bottom: SATA and IDE drives

Removing the covers

SAS Seagate Cheetah 15K.7 300GB 3.5″.  : Removing the cover was 7 outside short screws and 4 screws under the label.  Two of them were long.

SAS Seagate Savvio 10K.6 146GB 2.5″:  Removing the cover was 6 outside short screws and 2 screws under the label.

SATA Seagate Barracuda 7.2K.10 750GB: Removing the cover was 7 outside short screws and only 1 screw under the label.

IDE Seagate Barracuda 7.2K.10 500GB: Removing the cover was 7 outside short screws and only 1 screw under the label.

SATA Seagate Barracuda 7.2K.11 500GB: Removing the cover was 7 outside short screws and only 1 screw under the label.

First look inside each drive

Bottom Control Board

SAS Seagate Cheetah 15K.7 300GB

2 platters
6 screws to remove the center ring
bottom screw to remove head
4 screws for the horse shoe spacer

SAS Seagate Savvio 10K.6 146GB

2 platters
4 screws on center ring
I don’t have a small enough torx to remove the head
4 screws for the horse shoe spacer

SATA Seagate Barracuda 7.2K.10 750GB

4 platters
6 screws on center ring and 1 screw for the top of the magnet

4 screws for the horse shoe spacer

SATA Seagate Barracuda 7.2K.10 500GB

3 platters
6 screws on center ring and 1 screw for the top of the magnet
3 screws for the horse shoe spacer

SATA Seagate Barracuda 7.2K.11 500GB

2 platters
6 screws on center ring and 1 screw for the top of the magnet
4 screws for the horse shoe spacer