This sometimes happens on Windows computers when you have a program like Nero hogging your drive.
If that solution doesn't work (and it often doesn't), the easiest solution is to burn to a folder rather than directly to a disc.
Select this option from Share/Output/Disc.
Once the disk files are created, you can use your computer’s burner software to burn the VIDEO_TS folder and its contents to a disk.
As Steve points out, some programs can "hook" the burner at bootup, and tell he OS that it is not really a burner, but a removable HDD. This ARTICLE goes into more detail, and lists some other programs, that can cause this to happen.
Also, there were several threads on Dell computers having issues with a batch of multi-drives. In most of those cases, the users were getting SenseCode errors, but in some, the multi-drive/burner was just not detected. In those cases, Dell replaced the defective multi-drives. However, since you are not also getting SenseCode errors, I would proceed with thinking that software is the problem.