Apple - Clone Time Machine volume
It should work if you boot into recovery mode. Hold cmd+r when booting, then use Disk Utility to recover. If your drive is encrypted, you will have to mount it and unlock it first.
I experienced the same issue while restoring with Disk Utility (actually the best byte-by-byte method for cloning and migrating Time Machine disks).
And I finally found a way out (though not for Mojave?). Fortunately enough, because Apple Support's way is indeed a waste of (a lot of) time.
Turning off Time Machine doesn't appear to be enough, because MacOS does NOT consider its connected Time Machine disk(s) as any other standard external HD.
- Before restoring, you have to remove/disconnect the Time Machine Disk from Time Machine settings: Time Machine Preferences pane > Select disk > [name of your actual Time Machine HD] > Remove Disk.
• That makes the green desktop icon convert into a standard yellow disk icon, which corresponding different format doesn't abort the cloning/'resetting' process any more. This works fine in OS Sierra (with a Mojave Time Machine disk); but I just found out that OS Mojave can not be 'tricked' and gives the error message again ('improved' safety settings'???).
• As soon as cloning has been completed (after a couple of hours or even days) you can add the new disk name in the same Time Machine Preferences pane (or add both disks, if you want to generate a simultaneous backup).
Though, probably the easiest way for migrating a Time Machine disk is simply to clone it from another Mac computer. 'Foreign' Time Machine disks from other Macs appear to be treated automatically as standard HDs (also in Mojave? Or may this only have worked-out for me because of different OS-versions on both computers??? I just completed another clone successfully)!
Addition: Last night, SuperDuper! did an excellent job under Mojave to clone a Time Machine Disk (though file by file, and not byte-by-byte as Disk Utility does).
Success!