You may need to be more specific i.e. what operating system are you using, is it the external cd/dvd drive powered by USB. if so, is it USB 2.0 or 3.0) etc.
Also, do you mean your computer cannot detect the external hard drive?
If it is detected by windows, you should be able to see it in your device manager, usually found by right clicking computer > then choosing properties > device manager. If it has problems detecting the drive, then it could be one of a few things;
- Try a different USB port
- Try a different Cable
- The driver itself is not compatible with your OS
- The driver is not digitally signed therefore you will have to speak with the manufacturer and see if there is a driver for the operating system you are using.
If and when this is detected, you will then need to use something like Nero to write your data to cd/dvd's etc.
I hope this helps