In the context of computers, which term is correct? I needed to know today, and found a helpful explanation from Apple. Key quote:
A disc refers to optical media, such as an audio CD, CD-ROM, DVD-ROM, DVD-RAM, or DVD-Video disc. …
A disk refers to magnetic media, such as a floppy disk, the disk in your computer’s hard drive, an external hard drive.
Now, when you’re talking about your spine instead of your computer, or looking for the correct term in any medical context, disc is always correct.