I have a 27" iMac (2.8 GHz Intel Core i5) running OS 10.6.6. For external back-up I use a WD Studio II 1TB drive, partitioned into 3 volumes - named wd1 (data storage), wd2 (bootable clone of the iMac) and wd3 (more data storage).
When I woke the iMac from sleep today I saw that the third volume (wd3 - 480Gb) was not showing on the desktop. Neither Disk Utility nor Drive Genius could get the volume to mount. Drive Genius reported "Mount attempt failed for "wd3". Please Verify the filesystem, it may need to be repaired. (disk2s4, Operation not permitted 0xC001)" . Verification, repair and rebuild runs reported the volume as OK. The other two volumes on the drive are functioning OK.
Drive Genius's DrivePulse program identifies the volume as 'disk1s4', with 85% of its capacity used, but when I tried to run DG again the volume didn't show up in the list.
My question is, how can I get that volume to mount? I wondered if I need to update the Studio Book firmware, but if so, how is it that the other two volumes mount OK?
I'd be most grateful for any experienced advice!

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