Can you check the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\cdrom
At that registry key, in the right window, try changing the value data of Start to 3
That would change to startup state to "Manual" which I believe would be the default if there is no DVD/CD ROM.
The value data of Start may have been changed to 1 when the USB DVDROM was connected.
There is some general info in the following link on changing the "service state":
