Learn English – I’d Vs I’ll, what’s preferred

First off, I wouldn't lose any sleep over it. I imagine you could swap the two in almost any context and I doubt anyone (regardless of origin) would notice.

That said, for all meanings not concerning computers, "disc" is probably more common in British English, and "disk" in American English (that's the OED's take, and matches my experience), but use whichever you prefer. You might think disc would be older, since the word derives from the Latin discus, but there are some pretty old "disk" quotes listed in the OED (one from the Philosophical Transactions of the Royal Society of London, 1665--so no American influence there).

For computers, I have to concede that the Apple page does have a point (though calling it a "distinct difference" is carrying it way too far towards prescription on what's clearly a descriptive issue--for that matter, so is devoting a support page to it). One pretty much always sees "compact disc". "Floppy disks" (for those that remember them) were sometimes called "diskettes"; which term came first, I don't know, but the association between the two probably caused the K spelling to stick. Personally, I would also spell "hard disk" with a K as well--since the term originated by analogy to "floppy disk", that makes sense. I don't think the optical/magnetic split between disc/disk is based on anything like logic, but it does seem to be the way things have developed (mostly: I'd still call a CD drive a disk drive). The OED even acknowledges this: "Some writers have used the spelling disk for magnetic ones (‘hard disk’, ‘floppy disk’), and disc for optical ones (‘compact disc’, ‘laser disc’).". But then, it also lists "compact disc (also disk)".

There might be some technical applications where one is used over the other, but it'll never be based on any logic, since the two words mean exactly the same thing, so if you run into that, just look around to see which is more common. ("Disc" brakes gets the wikipedia entry, but no one would wonder for a second what "disk brakes" were.)

As to why both exist: who knows? Both spellings have been around a long time, so it is surprising neither has died out (since there never seems to have been a difference in meaning between them, at least until Apple got involved). If I were to engage in rank speculation, I would wonder if "disc" didn't come into the language directly from Latin, while "disk" meandered in via the French disque.

In the UK we would say Ready Meal not Ready Food. The dictionaries I have consulted include Ready Meal and not Ready Food.

Supermarkets will have a section labelled Ready Meals. These are complete meals (meat, sauce vegetables, starches) in one package, already cooked, and now displayed in chilled cabinets, not frozen. The consumer will take them home and reheat in oven or Microwave.

Convenience Foods are foods that are not necessarily already cooked but which can be prepared very quickly. For example canned foods, frozen vegetables or packets of soup noodles. As a generic term it can cover complete frozen meals, but also includes other quickly-prepared foods that do not in themselves constitute a complete meal, but often can be combined together to make something very quickly. Many UK households will have quite a few such convenience foods to hand.

Takeaway Meals are bought hot, perhaps delivered to the door, and consumed immediately.

Fast Food is bought on the premises of the vendor, and usually consumed there. It's fast in that it's not usually cooked to order. Examples: chains such as KFC, McDonalds, Subway. It is interesting that we don't say Fast Meals even though a complete meal is being provided; I have no explanation for why this is so.