Asked this question yourself? I certainly have. I have always used disc when referring to a CD/DVD and disk when referring to a hard drive but I never really knew if that was correct, until today, when I decided to look it up.
So for those of you interested
Disk: A disk refers to magnetic media, such as the disk in your computer’s hard drive or an external hard drive. A disk is always rewritable, unless intentionally locked. You can partition a disk into several smaller volumes, too.
A disk is non-removable.
Disc: A disc refers to optical media, such as CD-ROM or a DVD. Some discs are read only, some only allow you to write to them once and some allow you to rewrite to them multiple times.
A disc is removable.
Hope that clears things up, I found the definition here
-Matt