Time Machine - G-Drive message "appears to be read only"

Hi,

I have a 3tb G-Drive which I have connected via FireWire and is used only for Time Machine backups.

Periodically, every couple of days or so now it seems (is a desktop that is always on at home for iTunes, etc.) I get the message that the backup could not be completed as the disk appears to be read only.

I have used disk utility to repair the disk which did not work so next step was to erase and reformat which then worked for a couple of days.

The disk shows up fine in the finder, it is only Time Machine that can't use it for the specifed problem. I have looked under permissions and saw that they were read/write for myself and "staff" accounts but read only for "everyone". Upon changing "everyone" to read/write Time Machine again is able to use the disk.

Could there be something in Mountain Lion or my apps that is periodically setting the "everyone" to read only?

Thanks for any help.

The G-Drive is about 6 months old.