I also tried to follow Pondini's advice about a full reset (http://pondini.org/TM/A4.html) but the file com.apple.TimeMachine.plist was not there – yes I was looking in the correct place; plenty of other .plist files there. Accessed Library via alt-Go in the finder menu.
I would remove any such software (uninstall with the developer´s uninstaller). Might or might not solve the problem.
Running ClamXav from time to time is as far as I would go in that territory.
The sparsebundle is the huge backupfile made by Time Machine. If you choose to delete it, check Pondinis page: http://pondini.org/TM/12.html
Edit: First, of course, I would have created a bootable clone with Carbon Copy Cloner on a spare disk.
Hi Cozar, thanks for more help.
Sophos is gone, reboot, still no luck.
Am currently creating Carbon Clone of the bootable disk
It seems Pondini's/your suggestion is to delete the Time Machine back up using erase from Disk Utility...and then what? Use the newly erased disk for Time Machine backup?
I'm not sure why this would work when attempting to backup to a brand new external disk failed previously.
Went to my local Apple Store and met a 'Genius', no real joy there - also suggested cmd-R
Did cmd-R, no change, which really surprised me.
Then in one of those odd random moments of inspiration I tried deleting the com.apple... file and instead of trying to relaunch TM, I rebooted. And it worked straightaway.
Odd, relieved if somewhat puzzled!
Thanks for your help Cozar
I suspect a backup was already running (or trying to run) when you deleted the preferences file, so the new file got updated with the damaged info from the backup.
I've modified the instructions in #A4 to be sure there's no backup in progress.
It can be a bit hard to tell on Mountain Lion, since the Time Machine icon in the menubar no longer spins during the preparation phase -- you have to look at the Preferences window to be sure.
As a famous villian once said, "[Time Machine], you have failed me for the last time." (insert choking sounds of TM)
Following Pondini's troubleshooting steps worked initially (and thank you), but once again TM has failed and is stuck on the 'Preparing Backup' indefinitely. Apple is not dealing with this problem, and so, goodbye to TM.
After a lot of research, I've chosen to buy a backup solution called ChronoSync, as I need to keep archived versions of my stuff, not just a full backup like what CarbonCopyCloner or SuperDuper are designed to do. Those are also excellent choices but I need to have versioning and archives.
If anyone else is at their end with TM, might want to give this app a try. Good luck.