Elsullo2 installed a new, second hard drive into a desktop PC. But Windows doesn't see it.
Before we discuss your problem, let's consider why you would want a second hard drive in your PC. If you simply want to increase your internal storage, my advice below should help you. If you want to set up a RAID, see How to Set Up RAID on Your PC.
But please don't install a second internal drive for backup. The same power surge, malware, fire, or burglar that might deprive you of everything on your main hard drive will likely also rob you of your backup. For a better solution, see How Do I Backup?
But let's assume you just want to increase your PC's storage.
Your drive may simply need to be partitioned and set up. These instructions are for Windows 7, but should work with slight alterations in other versions.
Click Start, right-click Computer, and select Manage. This will bring up the Computer Management program. Once there, click Disk Management in the left pane, under Storage.
This will display all of your internal drives. The new one should have a black rather than a blue bar on the top. And it should be described as Unallocated. Right-click that drive, select New Simple Volume, and follow the wizard.
But what if the new drive doesn't show up in Disk Management?
In that case, shut down the PC, open it up, and make sure everything is properly connected. Remember that the drive needs a data cable (probably SATA) and a power cable.
If everything is connected properly, and the drive still isn't showing up, you may want to consider the possibility that your new drive is defective.
Luckily, it's probably under warranty.