How to unjam stuck iOS backup, or reboot when error occurs: "Finder can't quit because an operation is still in progress on an iOS device"

How do I reboot my computer when this error appears?

The Finder can't quit because an operation is still in progress on an iOS device

Or, how do I unjam a stuck iOS backup process, which is when the iPhone "Location" in Finder is stuck on this step forever: "Syncing (Step 2 of 4) - Backing up"?

And, how do I do it in the least-overkill way? (Avoiding overbearing state-reset solutions like hard-rebooting the computer or iOS device.)

Solution 1:

Well Neil, this is the best, least-cumbersome solution:

Abort the iOS device backup process by killing the AMPDeviceAgent process:

1. Open SpotLight by holding ⌘ and pressing Space, or by clicking the magnifying glass in the top-right corner of your screen.
2. Type "Activity Monitor" in to the SpotLight text prompt and press Enter
3. Find the process with name: AMPDevicesAgent and select it
4. Click the button that looks like an X on a street stop sign
5. Try clicking "Quit" first, and if the process doesn't disappear momentarily, try again by clicking "Force Quit"

In Finder, when the iOS device is selected from the "Locations" section in the left window pane, you should notice that the "Syncing (Step 2 of 4) - Backing up" message will disappear and get replaced by the multicolour

Unnecessary, cumbersome solutions:

1. Reboot or turn off the iOS device
2. Hard reboot the MacBook by holding the power button for a while

Non-working solutions:

1. Re-launch Finder by using Force Quit
2. Kill the MobileSync process, or any other process, from Activity Monitor
3. Clicking the "X" icon on the iOS device's "Location" entry in Finder (it does nothing)