For IDE interfaces accessed with CHS translations (i.e. ones over 8 gigabytes), it will be the software IDE implemtation that determines the number of logical heads and cylinders.

Unfortunately, we really don't know very much about your setup (I've never head of the ST 5100 nor was i able to find any information on ST's website (must be before my time?)) so we can only provide a limited amount of support.

Here is an interesting document: hxxp://www.seagate.com/support/disc/manuals/ata/d1153r17.pdf

That READ NATIVE MAX ADDRESS command on page 151 (pdf page 165) looks interersting, but it also looks like it doesn't necessarily work on all drives. If your API has a funtion to write a command to the ATA controller (or if you can directly control io), then that may well be what you need.

There seem to be quite a few documents on the internet that show examples of how to interface 8-bit microcontrollers with IDE drives, but most of them are not running on a RTOS and using an API... I would imagine some specifics on which API functions are available to you could be helpful to anyone else trying to help, as well.

Good luck!
-pnemeth