There is no real problem using HD disks as DD disks. The magnetic field reading HD disks is higher than DD disks, so the HD's can handle the change. I've been using HD's as DD's for years and had no problems with it - although I do know some people who have had problems.
The real problem is when you use DD's as HD's. DD's were never meant to handle the magnetic force that HD's are, so the DD's die very quickly.
