I'll include USB keys in this answer.
In a freespace erase eraser will first fill the drive and then erase the space, you would only need to use a 1 pass erase on a USB or SSD drive
For SSD Read this article and this - You need to consider the age of the drive and whether you need to wipe...
The actual issue is that you are trying to erase directories locked by the OS. Probably DLL's in use in that directory.
To wipe the C drive you really need to boot up with something like DBAN and wipe at the OS level.