I've now gotten Time Machine to see the older backups. Not sure if the first step is necessary as I noted that Time Machine Preferences took a while to update the "Latest backup" date, so it might have been enough to just perform a backup.
Step 1 - Verify the backups
- Following a guide on a similar topic I deleted the
com.apple.TimeMachine.MachineID.plist file (on my ReadyNAS this was
located under /c/.timemachine/MachineName.sparsebundle/)
- Have Time Machine verify your backups by having the alt-key pressed while clicking the Time Machine icon and choosing Verify Backups
At this stage Time Machine Preferences still states that I have no backups, but by choosing Enter Time Machine I can see the earlier backups.
Step 2 - Take a new backup
After this was done, Time Machine Preferences update both the Latest Backup date and Oldest Backup date to correct values.
Here is what might have happened:
You had a drive that was formatted as a PC drive and the Mac OS, thinking it was helpful, offered to back itself up to that drive. If someone clicked yes several times, that would prompt you to erase the drive (and since you didn't list the OS running on the Mac, the exact terms might be different if you have Lion versus Mountain Lion vs another OS version).
At that point, the drive would get reformatted, but the vast majority (perhaps all) of the files on the drive would be still intact, but no longer linked to a directory.
If the drive was ejected before it could write the first backup, you would have great luck using a commercial file recovery program. On Mac, you would get Data Rescue 3 or similar to undelete the files.
If the Mac couldn't erase the drive, that might be the best case for you since it wouldn't start backing up (which will begin to write new files on all the "free space" that it thinks contains no old files). If you see a empty disk with a directory Backups.backupdb - that implies that the format was successful and the Mac is ready to write files which will over-write free space and in time ensure that none of the old pictures are recoverable.
I'd say - get a hold of someone that has done drive recovery before and/or get a quote from a company like Kroll Ontrack so that you know what your options are.
The more data that gets written to the drive and the more uncertain you are about exactly what happened will increase the work needed to recover the files, but I would guess you only have to pay for software and time to get some if not most of the pictures back. If you performed the recovery yourself, you'll need a third hard drive to store the recovered files (or store them on the Mac if there is space) and the $99 software which you might be able to download and try for free.
If these files were PC formatted, you might have better luck with a PC recovery software, but I've used Data Rescue on many PC drives in the past and had good luck and not needed to send the drive out for professional recovery - so you may not need anything other than what I have listed above.
Best Answer
Your iMac has only a single disk in it, and that is the one you will be backing up. You cannot use the same disk to back up to. You need to get an external device to hold the backup data: either an external disk (that connects to your iMac via USB, FireWire, or Thunderbolt), or an Apple Time Capsule appliance (which also acts as a wireless network router). Once you have one of these, and it is connected to your iMac, you should be able to select System Preferences -> Time Machine pane, after you switch Time Machine on.