This could be an error related to Windows. Firstly, turn off the computer and disconnect all the devices (HDD and DVD-RW), then connect each device and start the computer. Once Windows assigns drive letters to the devices you connect, turn it off and start it again, but this time connect the next device. Do this until all your devices will have unique drive letters. Now you can perform any operations you need.
Additionally, go to Start > Control panel > Administrative Tools. Start Computer management > Disk Management. Right click the duplicated drive, choose Change drive letter, and pick another one from the list.