First is that the index table that points to all the data is corrupt. If so, you have to use software that will scan the whole drive and find the files one by one.

What I think it is though is the drive formatting. Windows uses NTFS, and OSX uses HFS. I'm not sure if Windows 8.1 can read the native OSX by default or if you need additional software to do it. I'm a bit lazy and I'm on my phone otherwise I would have looked more up before posting to verify my answers.