5GB Bootable Flash Drive - where / when?

I once cut corners on a HD upgrade as I dreamed that high capacity
solid state "HD's" were just around the corner - I would have been
happy with a 256MB.

Now we gave gigabyte flash drives, BUT... it was 1992 back then, and
now a full PC system + apps takes a few GB's.

I assume that multi-GB bootable flash drives exist for some special
high end application (say, cruise missiles or anysuchlike). I also
believe there must be at least some bizarre embedded computing devices
that keep their system in always-on RAM powered by reliabale redundant
buffered DC sources.  I know firsthand of several digital gizmos with
embedded OSs on lithum cell powered RAM as late as the early 1990s

QUESTIONS FOR THE WISER:

In PC terms, are they IDE, SCSI, PCI,
or some special gizmo in the northbridge neighborhood?

Any idea what they cost?

Any clue as to when we can buy a decent sized
HD-substitute, stuff it into a PC, and just
clone all the goodies into it?

BR + TIA

Filippo
trying to stop all the crazy spinning in Milan