Both spellings are correct — the choice depends on the meaning and the dialect. <em>Disk</em> is the computing spelling worldwide (<em>hard disk, disk space</em>) and the general American spelling. <em>Disc</em> is the British default and the spelling used for optical media everywhere: <em>He burned the film onto a Blu-ray disc.</em>
The core difference
There is no difference in meaning: both refer to a flat, round object. What varies is convention, and the conventions have hardened enough that the wrong spelling looks like an error in a given context.
Two forces are at work. The first is dialect — British English prefers disc generally, American English prefers disk. The second is subject matter, which overrides dialect in a few places.
Where each spelling wins
- disk — computer storage, everywhere in the world: hard disk, disk drive, disk image, running out of disk space, Disk Utility.
- disc — optical media, everywhere in the world: compact disc, CD, DVD, Blu-ray Disc, disc jockey. The formats were named with a C and the spelling stuck.
- disc — brakes and machinery in British English: disc brakes, harrow discs. American English writes disc brakes too.
- disc / disk — anatomy splits by dialect: British a slipped disc, American a herniated disk.
- disc — sport: the object thrown is a discus, and the flying variety is disc golf.
The test
Ask how the thing is read. If a laser reads it, write disc — CD, DVD, Blu-ray, and any other optical format. If the storage is magnetic or built into a machine, write disk — hard disk, disk drive, disk image. Both hold data, so the question is never whether data is stored; it is what kind of medium you are describing. That single rule handles the two cases people actually get wrong — hard disk, compact disc — and it holds in both dialects.
For everything else, follow your dialect: disc if you write British English, disk if you write American.
Why the split exists
The honest answer is that the industries chose differently and neither has any intention of changing. Computing standardised on disk early and kept it through floppy disks, hard disks and disk images. The audio industry named the compact disc with a C, and every optical format since has followed. Neither spelling is more correct; they are simply established in their own territories.
The common mistake
Writing hard disc or compact disk. Both are understood, and neither will confuse anyone, but both look wrong to readers who work in those fields — rather like writing program for a theatre programme, or the reverse.
The other slip is inconsistency within a single document: pick the pairing that matches each meaning and hold it throughout, rather than alternating for variety.
Frequently Asked Questions
Is it "hard disk" or "hard disc"?
Hard disk, in every variety of English. Computing standardised on the K spelling and British writers follow it too, even though they write disc elsewhere.
Is it "compact disc" or "compact disk"?
Compact disc. Optical formats — CD, DVD, Blu-ray Disc — all use the C spelling, and that holds in American English as well.
Slipped disc or slipped disk?
Slipped disc in British English; American medical writing uses disk, usually in the phrase herniated disk. Both refer to the same thing in the spine.