[zilexa]

Done. Now I see no contents in my harddrive in /media/UUID-of-the-hdd/ and also /media/usb0 is empty.

via ssh I tried sudo mount /dev/sda1, said it was already mounted. Checked back in XBMC and suddenly I could see all the contents.
Playback issue remains. it also takes a long time to start playing btw, longer than usual even for simple 175MB files which they do not stutter or freeze. Issue only seems to be with a little bit higher bitrate (like 720p tv rips).

I freed enough space on my laptop to copy the entire usb drive on it. Will do that now and completely format the drive as exfat (need compatibility with Windows, exfat is a bit more compatible then ntfs hopefully, since os x also supports it). Then copy everything back and report here. Will take some time..