[SOLVED] Can't access ext4 USB HDD

[pix3l]

Software
XBian version: Beta 2
XBMC version: current
Overclock settings: default

Hardware
Power supply rating: > 1Amp
RPi model (model A/B 256mb/512mb): B 512MB
SD card size and make/type: 32GB Class 10
Network (wireless or LAN): wireless
Connected devices (TV, USB, network storage, etc.): WiFi and USB HDD

Problem description:
Hi everyone,

I have a strange Problem:
I formated an USB HDD (1TB) with Ubuntu and put my media-files onto that disk.
I unmounted it and pluged it into my RaspPi.
The HDD is listed at Videos --> Files BUT I can't access it. It says something like "Path not found"

I have another USB HDD which is formated as NTFS and it works like a charm.

Any idea what to do?