I tried to connect the USB drive directly but it gave me some problems. I could format the drive, but when i made a diskcheck i got errors. The drive is powered by USB and i read that this could be a problem so i tried to but a USB hub in between. I can now format it, run checkdisk without problems. The backup also works fine but i get theese error everytime i try something. I only get them on mail, nothing in the webinterface.