Dear Vic, didyou try to reboot the Mac? Disconnect the drive and reboot. If that works tryto plug the drive again. If the Mac doesn't freeze try these next steps:
There's a 3step solution for your problem:
1. Download this software from Seagate:
http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=goflex-mac-install-software&vgnextoid=77002a...
2. If this software cannot create alink or communicate with the device, turn off your Mac and if you have anavailable pc try with it. If the drive works at the PC, Then is a compatibilty issue
3. Seagate report some compatibility problemswith this drive. If it is under warranty, ask for reimburse and choose anexternal HDD that has firewire 400, if your Mac is a recent model (2010-2011)you can buy an external drive with 800 firewire. If have doubt about the firewire option, lookfor about this Mac button and then press more info button, there you will findthat information.