hi.
the drive needs to be formatted first by Disk Utility as the default NTFS is not recognized by mac.
my seagate freeagent goflex 2T USB 3 was formatted to the mac format and works well with snow leopard.
i bought it in anticipation of the new macbook air.
i copied all my important files into it.
and when my new MBA with USB 3 arrived, i was shocked to find that it does not even recognize the drive.
but i found a workaround.
it worked when i booted into 32 bit and not the default 64 bit.
shutdown totally.
hold down "3" and "2"
then press power on but do not let go "3" and "2" until the desktop appears.
then plug in the disk.
then u find that the drive is recognized.
i tried to tell seagate the answer but logging into their forum is so difficult i gave up.
hope this helps.