Its really ridiculous that we cannot change permissions on folders and files, even when rooted. I was able to backup my N6 to my sdcard. But now, after a fresh ROM install, I cannot even read the folders to perform the restore. *sigh*....so frustrating. I tried all the solutions here, and have even been able to modify the platform.xml file directly. but titanium backup will still not see the backups on the sdcard. smh
If you can't read, its probably not a permissions issue. You may need to re-format the card.
See if you can copy your TB files to your computer, format the card, then put the TB files back on SD. Some folks have had issues with the M8 reading cards (need to reformat), but computers or other devices read fine.