By default, the ProjectWise Integration Module for Autodesk Revit automatically maps the drive letter R (assuming it is free) on the user´s computer to the user´s ProjectWise working directory, so that every user in the datasource who is using integrated Revit is automatically using the same drive letter. This drive mapping allows for Revit synchronization with the central file. The drive mapping occurs when integration is invoked, and is unmapped again when the session of integrated Revit is over.
If the drive letter R is not free for some of your users, then you will need to change the drive letter (using ProjectWise Administrator) to a drive letter that all of your Revit users have free.
To change the R drive mapping:
Picking a drive letter that is likely not used is the best option. Try "B", for example.
This is also not a setting you want to change after central files have been created.