External HD and Time Machine?

Yes.


When you format the external drive with DiskUtility, then set it up for TimeMachine and keep TimeMachine enabled when you disconnect the drive (or disable right before disconnecting and re-enable right after re-connecting) TimeMachine will resume its incremental backup schedule soon after you reconnect the computer to the drive.

Be aware that TimeMachine may take something called local snapshots during the time the computer isn't hooked up to its regular back up drive if you leave TimeMachine enabled.
http://pondini.org/TM/30.html

(After thought, may be redundant but added just in case. Don't forget to Eject the drive before unplugging it.)

I've forgotten to eject the drive many times and haven't had any problems, however it isn't recommended.

I use 1TB external HDD for time machine and works fine....and about reject the disk....better if u do it because u never know what u can lose from drive

deja-vu said:

Thanks for the fast reply. It sounds like this is the way im gonna go

What happens btw if i forget to eject the drive? If I know myself I might forget this once in a while :-/

Click to expand...

There is always a risk of data corruption when you unplug a drive without ejecting - resulting from improperly disconnected caches and other interrupted data transfers with the drive or unexpected disconnects from the controller on the external housing... By ejecting first you properly tell those processes to stop and thereby reduce that risk of data corruption. Try not to forget to do it.
This may save you from a confrontation with corrupt backups or an unmountable drive when you need that drive the most - i.e. when you want to restore something from backup. Which is the very reason why you set this drive up with Time Machine in the first place.
(The truly cautious will suggest a second independent backup system alongside these TimeMachine backups - to save you from multiple, simultaneous drive failures ...)