I need some help since i dont know much regarding Linux filesystems.

I have a 250GB USB disk connected to my Asus WL500 Deluxe with Openwrt. For some time now i cant copy new things to my USB-HD. The system says that the disk is full. However when i check it, the disk is only 50% full. If i connect my disk directly to my computer and check it with partition manager, i can see that the disk is about half full. The filesystem on the disk is EXT2, and in Partition Manager i can also see that about half my partition is ReiserFS. What is that? I dont know...

The disk contains 2 partitions, one normal and one swap on about 20MB.

Any ideas why or what i can do to fix this?

It sounds like you've created two partitions: one with ext2 and the other with ReiserFS. Either destroy partition two and extend the first or separately mount the second partition.

I did some playing around with fdisk to check partitions when the disk is connected to the router. All seems fine. One partition of 249GB and one swap of 1GB. The only thing i noticed was that the first partitions didnt have a disklabel of any kind. Is that relevant?

Is the 249GB partition primary or extended?

It is a primary. In case of emergency i can connect the disk directly to my computer so i can transfer files, but it is annoying.

What I don't understand is how one primary partition can contain two file systems. I'd think this would generally mean file system corruption. In any event, it sounds like something went wrong when the drive was initially partitioned. Since you should back up your data when messing with partitions anyway, I'd just start over once that's done.