Win 8 can't see files transfered from another OS

OK, I'll try to explain.
I installed W8 on a new drive to have a fresh install and to test it.
W7 is still on its original drive.
I physically plug in and remove the drives (SATA) accordingly, which ever I want to use on start up, to make sure they don't mess each other up.
I now started to move files from the W7 onto another drive (my backup drive D: )
all the files that were on that drive D: when I installed W8, are seen and accessible by W7 and W8.
But the new files, that I copy to D: from W7 are not seen when I boot with W8. Going back and booting with W7, they are all there.
I can easily transfer the files, by copying them onto a removable drive.

Why can't W8 not see them?

BTW, this happened before, but I wasn't too concerned about it at that time. Some time later the files were seen by W8 and I thought it was just a glitch.

Thanks for any help.