OK. If you look at ProDOS Technote #23 it describes the changes made
to ProDOS from version 1.0 to 2.0.3 (provided you find a recent version).
You can divide ProDOS 8 into 3 groups based on how it handles volumes.
A) ProDOS v1.0 to v1.1.1 - two drives per slot
B) ProDOS 8 v1.2 to v1.9 - two drives per slot except that slot 5 can have
4 volumes if slot 2 does not have a disk controller card in it.
The 3rd volume becomes S2,D1 and the 4th S2,D2. This remapping relies
on the disk controller firmware knowing what to do when passed a unit
number for slot 2 when it is in slot 5 - any controller supporting more
than two volumes must support this.
C) ProDOS 8 v2.0.x - up to 13 volumes on a SmartPort capable disk controller
(/RAM takes up 14th position S3,D2)
Note that the V2.0.x remapping can only use vacant slots - if you had a 1MB
memory expansion card in slot 4 which is a single volume (S4,D1) the remapping
code will not use the vacant S4,D2 for a drive from another slot. There is
a defined order that v2.0.x will allocate vacant slots based on a table in
the PRODOS file (list taken from v2.0.3):
S4,D1 S4,D2 S1,D1 S1,D2 S2,D1 S2,D2 S5,D1 S5,D2 S6,D1 S6,D2 S7,D1 S7,D2 S3,D1