The method is a bit dodgy but what happens is you connect a cable to the HD detect switch which will change the input from 8Mhz to 16Mhz when different disks are put in.
In pre TOS 2 I beleve that the computer will read the FAT but files will become corrupt. There needs to be a change in the access speed of the drive which is done by a small AUTO prog.
It has been years since I have done one so I could be wrong but that is simply it.
RE: HD switchable drive for STE