One of our users received this pop up window after they finished burning a disk using the windows disk burning utility. Bit of concern as to why the 3CX phone is trying to read from the disk drive?
The only thing I can think of, is that when installed, the programme was set to save any data (changes)to that particular drive.
You might want to go over the options, in the programme, to see if the D drive is listed anywhere. Otherwise, delete, then re-install, paying attention to anything involving drive locations.