Or just files that you synced?
Files that I synced or copied just work on the Apple.
If the files are in some proprietary backup system/app/database, well we recover them on the PC or Virtual PC (VirtualBox, etc.) and then we learn why we never use proprietary backup systems if we need to do what you asked.
I have a Seagate backup plus slim and I used to used to use it to save photos and music with my Dell laptop. I now use a macbook pro running El Capitan. Is there any way for me to retrieve those files I saved with Windows and open them with my mac?