Updating MS Office docs on an EHD on a Mac and Windows computer

I have an External Hard Drive, formatted with Windows a few years back. It now has a billion files in it. I have started using my Mac more frequently now, and need to continue editing Word, Excel files saved on the Hard Drive. But while I'm able to open the files I can't seem to save any edits using my Mac.

Is there an easy solution to make the files work on a Mac?