This isn't so much of an annoyance, but in the age of the internet it seems to be such a common error that it's worth a mention. Of course, 99.9% of people could not care less if the correct form of the word is used, but what kind of pedant would I be if I let it go unchecked? Answer: a very poor one.
So, disc vs disk - which is which? The answer is actually simpler than you might think.
A compact disc (CD) are read using a laser - that is to say, optically. A hard disk, on the other hand, is read magnetically. And believe it or not, that is it.
So, if data is reproduced optically, it's disc with a C; and if magnetically, disk with a K. Therefore, it's compact disc, but floppy disk.
I suppose this makes British author Terry Pratchet's Discworld series syntactically correct - since you read the books with your eyes!