[TonyDuell]

There was a company in England called PPL (Process Peripherals Limited or something like that) who made what was effectively an analogue video hard disk. The disk rotated 50 times a second (or was it 25 times a second, anyway, locked to the vertical sync rate) and (monochrome) video was recorded one frame to a track using analogue FM modulation (a bit like a video cassette recorder). The heads did not move to different tracks like on a computer hard disk, rather there was a head per track and you could electronically switch between them, thus allowing you to record short sequences (perhaps 1 or 2 seconds) of video and replay it.

I am told they were used for action replays and the like.

There was a version (which I have) that was used as a computer video display. A rack of boards that linked to a DMA interface on a PDP11 (DEC's DR11-B interface). You could create a video frame in the PDP11's memory and then record it on a track of the video hard disk, and repeat. And then replay the frames yo had recorded to a monitor. There were 3 sets of video electronics on this unit, so you could record 3 frames and then play them back at the same time to the R,G,B channels of a colour monitor.

And then semiconductor memory became cheap enough to use instead...