External hdd automatically unmounts after a couple of minutes

Hey guys,

I recently got my raspberry pi and just now tried to hook my ext. hdd to it for the first time. At first everything works fine, the hdd shows up, I can browse it, use it as a source and watch movies without a problem, but only for a couple of minutes. After that the ext. hdd just unmounts, with and a message in raspbmc shows up, that the ext. hdd was unsafely removed.
I have no idea what causes this problem, because the ext. hdd is working fine on my pc.

Some information about my setup that might help:
I'm using raspbmc on a usb thumb drive connected directly to the raspberries usb port, the other port is connected to a usb hub (which has its own power cord). The ext. hdd, which also has its own power cord is connected to the usb hub and is formatted with ntfs.

Normally I have to either restart raspbmc after the ext. hdd unmounts or reconnect it, but while writing this I tried to play an HD movie from the ext. hdd and it unmounted after only a few seconds into the movie, the movie then stopped and the ext. hdd remounted automatically. (before now I only tried sd movies). I also tried this a second time with the same results.

I hope someone knows more about this then I do and I'd be happy to post more information if needed, thanks.