My Backup Hard Drive now is read only


I have an iMac 27 inch running Mountain Lion. I have been using Time Machine to backup onto a 2 Tb G-Drive. For some reason, it is showing that my G-Drive is a read only drive. I went into Get Info to try and switch the settings, but even after unlocking it, they are grayed out. What can I do to make the hard drive read-write? Also, any idea why this just happened?

Thanks so much

Open Applications/Utilities/Disk Utility and run the Hardware test and Permission steps on that external. See if that helps.
I run into this from time to time.

The solution is to unmount/eject and re-mount the Time Machine drive (power-cycling the drive may help as well), or simply reboot your computer.

Haven't yet found a cause, but there are the solutions, for what it's worth.