Maybe combustion thinks the "home" directory is on your A: drive. Even if this particular project didn't have footage on the A: drive, maybe somehow the preferences got set there.
There are two ways you might be able to fix this. One is to delete the host.ini file in the combustion program folder to reset the various saved default folders. The other is to open the workspace itself with a text editor, search for anything that has A: in it and replace it with another drive letter.
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Greg Niles
combustion Product Specialist
Discreet