TIME MACHINE ON/OFF SWITCH MISSING

I have exactly the same issue (no slide switch and only the check box for Automatic Backup) after finally upgrading two units to Sierra -- one an iMac Mid-2011 and the other an Mac Book Air Early 2015. Both units have this issue. Had some initial issue with Time Machine hanging at "Preparing Disk" that was resolved by deleting Sophos Home off both machines (using Spotlight to search for "Delete Sophos" in order to use the deletion utlility), then doing a shutdown and restart. (My assuming is that Sophos Home and Sierra don't like to play together.) Backup updates are running fine....but the slide switch to turn Time Machine has gone missing since the Sierra upgrade. I've restarted Time Machine from Airport Utility (no change) and then did a hard shutdown (e.g. pulling the plug on Time Machine and waiting 10 seconds -- but no change). Firmware release 7.7 is installed on Time Machine, so that's not an issue, since I believe it is the latest and greatest. Tried un-mounting and remounting the backup disk from Disk Utility. Nothing brings back the slide on-off switch. So.....has Sierra done away with it? It was nice to have it, since sometimes you would want to stop backups on each machine.

Yes, I believe the "ON - OFF" slide switch was replaced by the "Back Up Automatically" check box in time machine preferences.

Absent the switch, how do you deal with a message asking to turn one of two disks in case of a conflict?