[gp2000]

You recall the disk capacity correctly -- 40 tracks, 18 sectors/track, 256 bytes/sector. However, uses track-at-a-time reads to squeeze 200 KB onto each disk.

Track reads are a bit of a devil's bargain as certain data patterns cannot be written and other data patterns cannot be read. It still means more data though there is some possibility that conventional sectors with tight inter-sector gaps could do the job.