Before you do anything with it get anything that is important to you backed up.
Can I ask why you want to take everything off it. What I would do is put it in its current state in your new desktop.
Copy everything important on it across to the hard drive in your new desktop and then put it in an enclosure to use as a back up drive for your new desktop. Thus you would want to keep all the stuff on it so you have it on both drives.
Only then should you look at clearing off the stuff you don’t want.
Apologies, I do not know your level of expertise so if any of this sounds complicated – get someone who knows what they are doing to help you.
Cheers
Danny B