Or copied files onto your Windows external drive that you couldn’t edit on your Mac?
The problem lies in the way in which the drive was formatted. By default, Windows devices use the NTFS file system. Modern Macs can read but not write to NTFS-formatted devices. By default, the Mac uses the HFS+ (or “Mac OS Extended”) file system. Windows machines, by default, cannot read HFS+ drives. Both Macs and Windows devices can read and write to drives formatted in the FAT file system, but FAT32 (the latest version of FAT) only allows for a maximum of 2 GB of data — enough for flash drives, perhaps, but no longer sufficient for most hard drive uses.
So, what can you do? Install software that recognizes the “foreign” drives.
An excellent list of tools you can use is found on MacWindows.com: