Drive letters will be wrong for existing projects

FWIW, the project drive and path assignment should not matter, if you are using per-project audio folders in their default locations under the project files. The projects use relative paths to the audio folders. If using the global audio folder, updating the preference in SONAR can also handle drive and path changes. The only case a drive or path change is an issue is if per-project audio folders are not in the default location. This results in projects storing fully qualified paths to the audio.
 
Still I can appreciate maintaining historic drive letter assignments to avoid learning a new layout.

Drive letters are an abominable relic from the pre-DOS CPM file system. Internally, Windows uses object identifiers (see https://technet.microsoft.com/en-us/sysinternals/winobj.aspx ).
 
I wish MS would remove these.
 
Be that as it may, the order that things get assigned are the order they enumerate from the Plug and Play subsystem which is based in BIOS and Motherboard issues.
 
In fact, there are cases where the drive letters may enumerate differently from Boot to Boot (see https://support.microsoft.com/en-us/kb/937251 ).
 
You can force the assignment of drive letters as shown above and you can mount drives without drive letters, but as folder locations, as like Unix mount points using the Computer Management -> Disk Management (Change Drive Letter and Paths...).
 
I have 8 physical drives made into 14 volumes and I have encountered lots of issues with this.
 
By the way, you can use drive letters A and B nowadays, as I do. They are no longer reserved for Floppy Drives.
 
When I mount ISOs, like when installing NI Komplete, I can run out of drive letters. You can't use UNICODE letters for Drives (but you can for folder names, if you want to get into trouble with the many programs that don't support Unicode). Also UNICODE naming sorts in weird ways, not lexically or in binary order.