I wish to erase free space on my default boot drive "Macintosh HD" using the function provided by Disk Utility. As I understand it, this process creates a file the same size as the free space on the partition and then proceeds to write to it as many times as specified (1/ 7/ 35). Since this slows down the OS when it is conducted whilst the boot partition is in use I decided to copy the partition to another HDD using Carbon Copy Cloner and then boot from that temporarily. However, since the back-up HDD is slightly smaller (500GB as opposed to 640GB) I was forced to use the non-block level clone option in order to exclude a couple of large folders. The other partition boots without any significant errors (just Little Snitch) but when I attempt to run the Disk Utility free space erase program, it throws up an unspecific error. This is in contrast to what happens when I run the same program from the boot drive and erase free space on the back-up partition, which runs without a hitch. Any ideas?