I've always been pretty mystified about the use of the word "disc" ending in "c" for CD's and the like. Why?? The world seemed to have gotten along pretty well with "disk" ending in "k" for a few centuries. Still no answer to that major conundrum of our age, but at least Apple has clarified the difference between the two in Knowledge Base article 30152:
A disc refers to optical media, such as an audio CD, CD-ROM, DVD-ROM, DVD-RAM, or DVD-Video disc. Some discs are read-only (ROM), others allow you to burn content (write files) to the disc once (such as a CD-R or DVD-R, unless you do a multisession burn), and some can be erased and rewritten over many times (such as CD-RW, DVD-RW, and DVD-RAM discs).
All discs are removable, meaning when you unmount or eject the disc from your desktop or Finder, it physically comes out of your computer. Disks
A disk refers to magnetic media, such as a floppy disk or the disk in your computer's hard drive, an external hard drive, and even iPod. Disks are always rewritable unless intentionally locked or write-protected. You can easily partition a disk into several smaller volumes, too.
Although both discs and disks are circular, disks are usually sealed inside a metal or plastic casing (often, a disk and its enclosing mechanism are collectively known as a "hard drive"). [Thanks to The Unofficial Apple Weblog]
A commenter said:
The spelling is most likely due to the fact that Compact Discs were invented by a task force led by Philips (a European company), whereas hard disks were developed by IBM (an American company, naturally).
Investigating further, I found:
In British English disc is the usual spelling, but American English uses disk, and disk is also more common in computing, as in disk drive. [Tiscali.reference]
Now I will rest easier at night, knowing that this momentous mystery has finally been resolved.