It will be 90% complete, but, given the way that the file system works, you will have a not much more than 0% increase in security
The process has to be restarted from scratch. Assuming that you stopped the erase cleanly, the erasing files will have been deleted, and have left unused space. It is really necessary that the unused space erase does the whole lot in one go.
Did you ever do the write up on each erasure method with a brief description of what each is and does? I mention this as I found a forum post yesterday where someone had requested it and you replied that you may look at doing it.
I'd be interested to know what they are, time to complete for a given size HD as a yardstick and the differences if any.
I've used this software 3 times now and I think Joel and yourself do a great job. I just wish I understood it a bit more.
If you erase a file directly, there is no point in running a free space erase to clear the same data a second time.
If you are clearing a drive (and it isn't a system drive), the best thing to do is format it (quick format will do), then run a free space erase. To clear a system drive, you have to connect it (by internal mounting, caddy or dock) as a non-system drive to another machine and erase it from there.
Free space erase takes a long time, and can stress the drive, so is not really suitable for, say, daily use. In normal use (e.g. to securely empty the Recycle Bin), I use file erasing. If I'm maintaining a system, I might use a free space erase (perhaps after defragmenting the drive).
First/last 16K erases what it says. The file is made unusable, but not completely erased. Clearly not a good idea for, say, a sensitive database file. Remember that disk/sector editors can read the drive without help from the file system and can find non-erased data, even though they may not be able to determine which file it comes from.