People at the store know NOTHING - especially when they say they do.

So when you go to the store and explain to them that you want an external hard drive and the other one you have stopped working and even when it did work it did not go on your apple even though the last people said it would and the store people tell you all the ones we have go, you just plug them in and no special measures required... and you tell them that is what the last people said.. but they also said that it it best to plug the external into the apple first before the pc, and the people at this store tell you that is rubbish and it's actually the opposite, you plug it into the pc first and then the mac..Which is useless to me as my pc is not working at all hence the reason I need an external as I have no where else to put my hoarded files...
ANYWAY my advice is , don't bother wasting your breath because these poeple don't know anything.

Don’t listen to anything the people tell you at the store.
Don’t do anything they said

here is what you should REALLY do.
Go to the store and buy the external. Bring it home and plug it in and notice that you can see the drive but you can't copy anything onto it.Then do this stuff which is the 200$ worth of advice I got from the apple store on the phone, for free, coz they rock!

Making a external hard drive work on a mac

Plug in external as directions state
Open finder window and see hard drive just sitting waiting for you to put all your stuff on
Open disk utility and click on main title for hard drive the shop told you would work fine without any special measures
Start special measures
1. select PARTITION
2. under volume scheme select 1 partition
3. next to format make sure it says Mac OS extended (journaled)
4. Next to name put something smart like “Shinestuff” coz , well, you can
5. Click partition
A warning will come up but you don’t need to panic coz the external was useless to you until you do this so DO IT.

You can now use your external to put on all your horded files that you want to keep so you can get a whole bunch more!!!*giggles*

EDIT: Note from future me (7th May 2011) I so rock for leaving this note for myself! This time I got a 500gb and this blog post was like exactly what happened again! This time I created two partitions which load like two drives. Thanks me, we rock!