RAMAC, not RAMDAC: https://en.wikipedia.org/wiki/History_of_IBM_magnetic_disk_d...

However it doesn't seem to be divided into sectors at all, more like each track is like a loop of magnetic tape. In that context it makes a bit more sense to use decimal units, measuring in bits per second like for serial comms.

Or maybe there were some extra characters used for ECC? 5 million / 100 / 100 = 500 characters per track, leaves 72 bits over for that purpose if the actual size was 512.

First floppy disks - also from IBM - had 128-byte sectors. IIRC, it was chosen because it was the smallest power of two that could store an 80-column line of text (made standard by IBM punched cards).

Disk controllers need to know how many bytes to read for each sector, and the easiest way to do this is by detecting overflow of an n-bit counter. Comparing with 80 or 100 would take more circuitry.