Question Can I use my old HDD with an OS installed on a New PC which also has Windows 10 installed, without having to format the drive completely?

[Shrixh]

So I built a new PC where I installed windows 10 on an SSD. Now after I built the PC I found a 1TB HDD in one of my old builds. Can I add that old HDD onto the new build and use it as storage? The old HDD also has windows 10 installed, is there any way that I only have to format the C drive and save the data on other drives? I mean I need the data in the D and E partitions of the drive. If yes how do I do it?

 

[USAFRet]

So I built a new PC where I installed windows 10 on an SSD. Now after I built the PC I found a 1TB HDD in one of my old builds. Can I add that old HDD onto the new build and use it as storage? The old HDD also has windows 10 installed, is there any way that I only have to format the C drive and save the data on other drives? I mean I need the data in the D and E partitions of the drive. If yes how do I do it?

You need to wipe that old drive of everything on it, and continue with a blank drive.
"Everything" = more than just the C partition.

 

[popatim]

I have to disagree with you USAFRET.

Install the old drive.
Go into disk management and give the partitions new drive letters by rt clicking the partitions in the lower part of the window and selecting 'Change Drive Letters & Paths'

Keep in mind programs in those old partitions likely won't work but the data should. For example, all your photos, videos, documents, spreadsheets... should all work fine but things like Office won't because the new OS never had them installed.

if you have games on those other partition those might work if they used a launcher app, like Steam or Origin. All you need to do is add the old games location as a new library location in Steam/Origin.

 

[USAFRet]

OK, yes.
If that drive has multiple partitions...former C, D, E...if done carefully, the D and E can be retained.