External hard disc not ejected properly
It's one of the major flaws with newer seagate drives (alongside no on/off switch so you have to pull the power cord out the back and risk damaging the disk). They are just not compatible with Macs unfortunately. They go to sleep and there's no way for the Mac to reconnect to them. I've contacted Seagate about this and they said "Sorry there's nothing they can do about it, perhaps I can contact Apple and they can do something about it". Great..!

I basically don't buy Seagate any more and don't recommend Seagate to any friends or family with Macs. They just cause too many issues.

I'm using 10.6.8, I've not tried 10.8.3 as poster above suggests.