With the Mac, check the "permissions" on the hard-drive found in the Info window (Cmd-I). You might want to run the drive through the Mac's Disk Utility app and "repair permissions" from there. I'm guessing the drive is formatted for FAT32, which almost every computer on the planet can read. I'm not sure if Disk Utility's tools will all work on FAT32, but it won't hurt to try.
(If you were handling files over 4 GB, you could also use a drive formatted for NTFS. The Mac can read NTFS, but not write to it—at least not without third party help. NTFS-3G is one option and Paragon NTFS is another.)
If that works, you can at least copy the files and worry about making the two systems talk later. You might consider an NAS (network attached storage) drive. I know that Win7 "broke" the easy connectivity of WinXP. I haven't touched Win8 yet (ha-ha).