Ok, that explains why it reboots when the hdd is plugged in. So my solution is a powered usb hub.
Thanks for the help

When it is connected, you can hear that it tries to spins up, then it makes a click noise and then it tries again to spin up (It's not broken. Works fine on my pc). It must be the power that is the problem, but why can the raspberry pi not give enough power, even with a 2A power supply?

I have it connected directly to the raspberry pi. Is it because it does not get enough power?
I have seen a video where the same kind of hdd is connected directly to a raspberry pi.
Link: http://www.youtube.com/watch?v=0Vc6Jtoi8PA

Why does this person not a have any problems?

Edit: A few moments after it unmounts, it mounts again and work for some time, then it unmounts again.
It keeps doing that.

When i connect my hdd to my raspberry pi it connects and works, but after a few moments it stop working and unmounts. When it unmounts it also make a click noise. When connected to my pc it works without a problem. My hardware is the following: Raspberry Pi Rev 2 512mb WD My Passport 500gb Hope anyo...