Seagate may have included some special software that requires 10.6 and above to function.
Plug the drive into the Mountain Lion machine, launch Disk Utility and format the drive with one partition, Mac OS X (extended) and check that the Options box shows GUID.
Now everything on the drive will be erased and the drive can be used with all of your devices
I bought this but have not opened because it says OS 10.6 or higher. I have 2 iMacs running 10.5.8 and a laptop with Mountain Lion. Does anyone know if it will work on all these machines? Want to use time machine also.