That was strange ... I have a external USB drive for my TimeMachine Backup. My iMac wan't able to see it since I moved it to the other side to have some more space on the right side. Sometimes it showed up as corrupted in DiskUtility but when I tried to "Repair" it it was gone again. It was like that for the last 40 days. I was just fiddeling with it again and saw that the USB cable was a bit loose. I tried another cable and suddenly everything works! That is really the first time I had a problem like this.