The disk drive was actually used as memory of the
D-17B (and also as
the storage for the registers I believe, but I can't remember, and
BitSavers has no docs for the D17). But it essence, the D17's disk is
similar to the disk of the Autonetics RECOMP II (which DOES have
documents on BitSavers), so for a better general idea of what the disk
was used for, go check those documents.
I would expect that the disk drive also has at least one pre-recorded timing track that
provided some clocking signal(s) for the machine. It was very common in the bit serial
disk/drum-based main memory computers to use one or more tracks with pre-recorded timing
information on them to generate some or all of the clocking for the rest of the logic.
This was a convenient source for clock pulses that were synchronized with the various
other data recorded on the disk/drum, and also helped correct for the small rotational
speed variations of the motor that spins the disk/drum, since the clock pulses governing
the operation of the machine skewed along with the data.
As such, there probably is some "content" on the disk that is required to get
the machine running, in the form of clock pulses.
Of course, if the characteristics of the clock signal(s) are known, it's not a complex
matter to reproduce them with external logic.
The high school I went to had a computer in the computer lab made by 3M (the same people
that make Scotch Tape and tons of other stuff). It was a process control machine that was
donated to the school by Natural Gas utility that used it to monitor and manage gas
pipelines. It was a 24-bit, bit-serial architecture, with two main registers (A and B),
and main memory was a magnetic drum that held 8K words. Opcode was 5 bits, and each
instruction had both an operand and next instruction address. Disk was organized as
block/track/sector, so addresses were formed this way. The drum provided a master clock
signal that was used by the rest of the machine for timing. The motor that drove the drum
was an inexpensive motor much like that that drives a sewing machine. There was some
phase-lock circuitry on the clock signal to let the machine know when the drum was up to
speed, which would then make the machine go "ready", but there was no active
feedback to control the speed of the motor beyond that. If the clock rate from the drum
fell outside a window of tolerance, then the machine would lose ready state, and freeze
where it was at. The drum was belt driven and one time the belt broke, and the machine
froze. I was able to remove power from the motor, put on a spare belt (looked a lot like
a vacuum cleaner belt), re-apply power to the motor, and once the drum came up to speed,
the machine resumed what it was doing without a hitch.
-Rick