AFAIK "Disk" comes from diskette, but i'm not really sure of it. At least for floppy disks and 64DDs it would make sense, I think.
High-Definition Digital Versatile Disc (HD DVD) or Blu-ray disc, yeah. They're all optical disc storage media.
It's the good old British spelling of Disc, the Japanese (and Europeans) who made most optical medium called it Disc for that reason but the Americans with their work on magentic media called it with their corruption of the Queens English and called it Disk. Still if we all call it Disc then maybe it will change.
z_killemall is correct, you have Disc's and Diskette's, a CD/DVD is a disc, a floppy/64DD is a disk. I would guess you would differentiate by it having a caddy of some sort or not. A Hard Disk Drive and a Digital Versatile Disc.
What's the etymology of 'disc' anyway? I know 'disk' is a proper term that also refers to the shape in general, I always thought 'disc' was some fancy "modern" moniker.
There's lots of underlying etymology; some starting points are here: http://dictionary.reference.com/browse/disk Of course both terms come from the latin "discus", but the free online dictionary of computing adds this: disk storage 1. magnetic disk. 2. compact disc. 3. optical disk. Note: the american spelling, "disk", is normal for most computer disks whereas "compact disc", having come to computers via the audio world, is correctly spelled with a "c", indeed, this spelling is part of the CD standard. (1995-07-30)
Yes, for optical media I've always used Disc, I have always thought that Disk is for floppy disks and magnetic media in general.