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I want to take a HD out of a Dell OptiPlex 790 and place it to a newer Dell OptiPlex 9020. Is this possible and if so, what pre - reqs will need to be done, if any?

If you want to use it as a backup drive, thats no issue. Even if your not, it wont make a difference. Depending on the size of the computer will determine if it will fit. At most all you need is a sata cable which you can take out of the old computer. If you have the tower it should be fine, if you dont, check to see if there is space for mounting another hdd. If everything works, go into windows, go to start menu and type in Disk Management. Then pick the one that you added and format it. If you want to get any data off it do that first and then format. Hope this helps and sorry if it was confusing!

Swapping out drives between systems that both use SATA drives should be doable. But! You do need to keep track which way you go! An older SATA I (1.5 Gb/s) or SATA II (3.0 Gb/s) drive will work in a newer SATA III (6.0 Gb/s) system. Sadly, a Fixed speed SATA III (6.0 Gb/s) drive won’t play in either a SATA I (1.5 Gb/s) or SATA II (3.0 Gb/s) based system. Now the Rub! As it turns out Some HD’s have Auto SATA port speed sense technology and Will allow you to move a SATA III (6.0 Gb/s) drive in to a slower SATA I (1.5 Gb/s) or SATA II (3.0 Gb/s) system! You’ll need to review the drives specs to see if this is possible. While that answers the hardware level issues you still have the OS level issues to contend with. The problem is with the installation of Windows as the OEM release you have on each system may not have all of the needed drivers to work in the other system so you may need to re-install the OS to update it with the needed drives for your other system.

When you install Windows onto one computer, it configures itself for the hardware. Windows is copy-protected. It won’t work on hardware too different from what it was installed on. If you have a retail version of XP, that won’t be too much of a problem. But if XP came pre-installed on the old PC, that’s the only PC that copy can run on. You’re better off buying a new hard drive and Windows–or buying a PC with them included. Just make sure you migrate your data files to the new PC and properly remove them from the old one before you get rid of it.