Unable to complete backup. An error occurred while creating the backup folder.
first 27 imac mountain lion 10.8.3 external usb drive toshiba 1.5 tb so far with apple care have unmounted and mounted drive, reformated drive guid, verify and repaired drive nothing to repair though, reset permissions on imac, reloaded time machine, tired different usb ports on machine, tried powered usb hub, did Pondini troubleshooting per guide, tried external usb WD dirve. set hard disks to never time out, waved garlic over monitor and incense. All of the above solutions work for initial try after running them then second or third hourly get the same message again. have even used finder to initiate disk drive and then tried time machine while it is lit up, same error. less than 30 days left on apple care seems to be a software issue, spnning drive every few minutes seems like a real kludge, and in my experiment probably not a work around either. Suggestions?
imac i5, Mac OS X (10.6.4)