Being a multiuser operating system, OS X sets up access permissions for all files and folders on the system, including external drives, which are merely accessed as a folder once attached and mounted. Since permissions are set up through account UUID and UID numbers, if the drive is used with different systems, it may have permissions associated with it for accounts that your current system does not recognize, or even odd permissions settings that can prevent access to the files on the disk or to the disk itself.
This may happen even though getting information on files and folders shows you ought to have full access to the drive.
To prevent such errors from happening, OS X includes a setting to ignore permissions on external drives, so all files on the drive should be fully accessible regardless of their permissions settings.
To set this option for external drives, select the drive on your desktop or in the Finder sidebar, and then press Command-I to get information on the drive. In the information window that pops up, go to the Sharing section and click the lock to authenticate. Then check the box to “ignore ownership on this volume,” or toggle it off and then on if it’s already enabled.
If this setting is enabled and you still do not have access, you can try removing the system’s volume information database, which holds this setting for external drives on the system. To do this, choose “Go to Folder” from the Finder’s Go menu, and then type in “/var/db” and press Enter to open the hidden system database folder. In here, locate the file called “volinfo.database” and remove it from this folder (you may need to authenticate to do this).
Once this is complete, detach and reattach your external drive, and then try toggling the setting to ignore ownership on the volume.
Note that these permissions settings will only be available on drives with formats that support them, so if you are using a FAT32-formatted drive, the option to ignore permissions will not be available.