Re: I'm curious...
The GPIB, originally known as the HP-IB (Hewlett Packard Interface Bus), was invented to tie automated test equipment together. Later, it was bastardized into a peripheral bus for small computer systems. I've seen it used on everything from Mainframes to the Commodore PET.
HP used it for sorts of things including for the disks. For the interfaces which followed the standard (IEEE-488) they were limited to 350kB/s but as you say they bastardised the handshaking to get a fast version so they had a disk interface version which could manage the heady realms of 1MB/s :-) but then we were talking about the days when a 7908 was a 16MB disk which was about the height and depth of your desk and about a foot wide IIRC. Ha, guess I'm getting old coz I can remember things like that but not what I had for lunch yesterday.
HP used it as their disk interface for ages until it was killed off by SCSI. They did have their own fibre based disk interface as well towards the end which offered the same performance as SCSI but much longer "cabling".
I bet I've still got some HP-IB cables somewhere.