Do you have an additional internal or external disk (USB pendrive perhaps) attached to the machine that wasn't attached when the OS was first installed?
The hardware 'enumerates' all possible disk channels and devices when initialising and this is what is reported to the OS (when it installs).
Clearing the CMOS (by virtue of removing the battery) would initiate this action and an additional disk device may 'shuffle' the order of the existing allocation. The numbering, from 0 up, is hardware type specific and the order 'logic' differs (slightly) between manufacturers.
'rdisk' refers to the physical hard disk.