You have just finished installing a new hard disk in your computer. Now the DVD drive won't work. What should you do first?

The most appropriate first step after installing a new hard disk and finding that the DVD drive is not working is to ensure that the DVD drive cables are properly connected. When installing new hardware, it's common to inadvertently disturb or disconnect existing connections. The power cable and data cable for the DVD drive could have been loosened or accidentally unplugged during the installation process.

By checking the cables first, you can quickly determine if a simple physical connection issue is the cause, which is an easy fix and less time-consuming than the other suggested options. If the cables are secure and the drive still doesn't function, further troubleshooting such as checking for driver issues or updating firmware can be considered. However, addressing the physical connections is always a logical and necessary first step in troubleshooting hardware issues.