Disk vs Disc
The best definition I have seen is that you use disc for the unprotected discs you can hold in your hand. Compact Disc, Blu-ray disc, etc. When talking about fixed disks, you should use a k in there somewhere. Hard disks, floppy disks (diskettes), etc.
I do not remember seeing "hard discs" twenty years ago. This spelling emerged (AFAICT) when Compact Discs started making their way onto the PC. Only at that point did the new spelling catch on in some places.
Another thing that confuses people: k = metric kilo (1000). K = bastardized Kilo (1024 -- 2^10). b = bit. B = Byte. When discussing speed, one would mostly count bits in a metric fashion. Hence 14.4 kbps = 14400 bits per second, not 14.4 * 1024 Bytes per second. Hard disk manufacturers (most of whom, oddly enough, are capable of the correct spelling) still prefer to use kB though so that the drives look artificially big.