the 320 is a 20 MB, ST225 workalike...
According to the various sources I've been able to find, it
has 615 cylinders and 4 heads and is indeed a 20meg drive.
Jeff
--
Collector of Classic Microcomputers and Video Game Systems:
Home of the TRS-80 Model 2000 FAQ File
http://www.geocities.com/siliconvalley/lakes/6757