Help recovering EXT2 formatted disks

Hi I'm new here and I have done something really stupid. In an attempt to test the performance of a couple of drives on my Debian server I ran the following commands....
dd if=/dev/zero of=/dev/sda bs=1000000 count=512 dd if=/dev/zero of=/dev/sdb bs=1000000 count=512 dd if=/dev/zero of=/dev/sdc bs=1000000 count=512 ...not very smart since this wiped out the first 512mB on each of the drives.
I have managed to recover most of the data on the sda drive using Stellar Phoenix Linux recovery but I have problems with the other two
sdb: This is a 1,5TB Western Digital WD15EARS with GPT if I'm correct and EXT2
I guess I need to try and recreate the GPT tables to recover it?
Stellar finds nothing on it...

sdc: This is a 2TB WD20EARS with a normal primary partition and EXT2 fs
Trying to scan this disk with SP Linux recovery the software crashes.

I have had these up with Stellar Phoenix support and after some emails back and forth they said I needed to send both drives to them for diagnostics.

I have access to r-studio 4.2 at work could this maybe to the trick? I'm a bit of a noob when it comes to hard drives so I need all the help I can get.