Data recovery...

Ok here is the story... I had an older 200 GB firewire/usb 2.0 HDD. I ended up getting a new 400 GB one and when I got it home i hooked it up to my mac. I figured i would have to format it but much to my suprise it just popped up as "Seagate_Disk" and had the firewire drive icon. So i copied everything from the 200 to the 400 and thought it was ok to delete it off the 200. Well when i restarted my computer the next day it said "the disc you inserted is not readable by this computer" I tryed plugging it into my PC and it says would u like to format this disc. I know the data is on there but i cant seem to get any computer to recognize it. I was curious what the best data recovery software is for the PC or OSX. Or is there a different way I should go about getting my data back?