Question What should I consider before deleting files from old system drive now used as storage

[Smoss]

I upgraded to a new built and installed Windows 10 on a new SSD. I hooked up my old main drive to the new build and now want to delete all the old files I don't need or are not used (Old Windows and games and programs that I have reinstalled on the new SSD and won't be accessing on the old hard drive. What is the best thing to consider before deleting these files and what is the best way to delete them? I don't want to delete anything I may wind up needing in the future.
Thank you in advance.

 

[Alabalcho]

With storage being cheap these days - if you don't need that space, leave it for now (may be move it to a folder with easy-to-recognize name). After six months, you can delete them.