As restarting the computer didn’t change anything, I looked up the problem at Google and found a lot of other users with a similar issue like mine. It seems, this is problem with Windows Vista, Windows 7 and also Windows XP, which might occure when you use the 64bit version, and can’t be really reproduced. It’s kind of a random thing, that appears out of nowhere. Some state that it has something todo with backup-software blocking the disk, others say it might be virus-protection or corrupt fileheader.

open diskpart

Anyways, I figured out a solution that worked perfectly and doesn’t even need to restart the system, using the tool Diskpart, that’s included per default since Windows Vista. Therefore you have to open a command prompt (WIN + R), type diskpart and hit Enter.

remove readonly flag

Next, you’ll need to figure out which volume number the partition has that’s protected. To do this, use the command LIST VOLUME. Now, switch to the required partition with SELECT VOLUME #, where # is the ID of the volume. Finally, simply remove the protection with ATTRIBUTES VOLUME CLEAR READONLY. That should do the trick!