Is there any reliable way to backup the contents of the aTV to an iMac and later to restore the aTV from there?
If you have the FireCore software installed then there is a backup/restore option under the Maintenance menu. This does the backup/restore via the Internet.
Thanks but I was hoping/looking for a backup/restore option for the total system. Bit similar to making a clone.
That is not available - the technology does not allow it. This is why you will see mention of getting the SHSH blobs for you ATV2 system so that you can restore a particular release of Apple firmware even when Apple have stopped signing it as one always needs to start with a Restore via iTunes.
If you have done a restore of the basic firmware via iTunes; reinstall the FireCore software via the FireCore installer; and then use the Firecore restore option you have achieved the same effect. One advantage of this approach is that it still works even if you upgrade the Apple firmware or FireCore software to later releases during the process.
That’s indeed a good reason to go the mentioned way. Thanks for the advice.