Nah this is totally easy to solve. Worst case is you google "change drive letter".

Oh wait, I just did it.

The only nag you may have to do is temporarily change the burner to something else to "park" it whilst you change your other drive, then complete the process (ie, switching drive letters). But still, not rocket science.

Ok, I changed the drive letter work great. thanks guys.

It seems like you've solved your problem BUT:

It seems very unusual that SONAR should react this way unless some of the installation files were on that other drive.

By default SONAR wants to install everything to your C: drive so in case you changed something this seemingly resolved problem may come back to bite you in other ways.

I'd have a close look at where all my SONAR associated files are ie besides the project and other audio files. Perhaps you have your picture cache Folder on that external drive and that was the issue. Picture Cache folder should always be on the same drive as your SONAR program.