Why did drive wiper start over?

My PC keeps crashing, and tech support tells me I have to return it. So I'm using disk wiper to wipe my PC. The problem is because it keeps crashing, it's having a hard time getting through it. Finally, it got through to 100% on the C drive, and then it automatically started over, this time telling me exactly how many files it's gone through at a time. And this time it's going to take four more days. The first one through took about 7 hours. During that first pass, was it even writing over my files? If it doesn't make it through to 100% again, has it done no good for me?

Possibly a disk error causing it. Rather this helps or not try running chkdsk /r on the drive using either Command Prompt or Windows PowerShell with Admin privileges.

Depending upon the drive capacity it can take several hours to complete.