Forgotten what it was called now, but I had a snapshot type device that dumped the Spectrum memory to the Sinclair microdrive.
As this could work at any point in time, it meant you could also save your game progress, as you just hit a button, and it froze the system. You could then dump the memory state to the microdrive at that point in time, mid game, and reload again later.
Hmm, it's going to bug me what the thing was called now!! Anyone any ideas?
It was a small box, plugged into the rear expansion, and had a single red button on the top. Hit that and the speccy froze, and you got an on-screen menu that let you peek and poke memory, save/load to/from the microdrive, and various other bits.