External backup disk fails regularly

I have an external 4TB G-DRIVE disk for time machine backups on my late 2010 21.5" iMac.

OS/X Mountain Lion fully up to date with all Apple-supplied updates installed as of January 29, 2013.

Originally the external G-DRIVE disk was pre-formatted as a boot disk. I erased it, and re-partitioned it as a GUID partition.

When I run the time machine backup, it generally fails with this message: "Time Machine couldn't complete the backup to "time machine backup" (that's the name of my disk). Unable to complete backup. An error occirred whie creating the backup folder."

I open the disk utility and verify the disk. If there are any verify errors, I unmount the disk and run the disk repair utility.

Once the disk repair is successful I mount the backup disk and re-open time machine preferences.

The "Select Disk" routine finds two versions of the same physical disk.

When I select the first one in the list, it prompts me to remove the disk.

I generally cancel this option and select the second version of the same physical disk in the list.

It prompts me to "Use this disk".

I then open the little time machine backup icon and select "Back up now".

Sometimes the backup completes successfully, sometimes not.

Clicking on the "?" symbol in the error message window opens a list of "Time Machine Problems" on the apple support web site.

The various remedies about robust cable connections to the external disk are checked and verified as OK. I'm on a firewire 800 cable from my iMac to the external G-DRIVE.

As the external disk is not shared, I have not set a password on it. I do not recall an option to set a password in the disk utility, but perhaps I missed it, as I was not concerned with putting a password on it.

My expectation is that the external disk can be selected once, then run its hourly time machine backups with no further intervention.

The reality is that sometimes it will run a few backup cycles, then will fail with the "time machine couldn't complete the backup...." error. Then I start over again, sometimes I shutdown and reboot, open time machine preferences, re-select the disk (the same disk appears as duplicates in the list, as described above.)

A side-effect of this bizarre behaviou is a mix of curiosity and annoyance on the part of the computer owner.

The external G-DRIVE disk appears to go to sleep after a few moments of inactivity, and sometimes wakes up OK, sometimes not.

I have not found any physical or software switches to this effect in the disk or the technical literature about the disk.

Does anyone have similar experiences with an external G-DRIVE disk used as a time machine backup disk?

AND, does anyone have an effective remedy that fixes it?

I have checked the permissions on the backup partition, and they appear to be OK and logical.

The format is Mac OS extended (case-sensitive, journaled)

Owners Enabled: Yes

Mount point: /volumes/time_machine_backup

Capacity: 4 TB, Available 3.6 TB, Used 403.8 GB

Many thanks

Jim Taylor

Winnipeg, Canada