Unable to copy existing Time Machine backups to new disk

I understand Apple's instructions aren't satisfactory and DU's Restore function is complaining too.

Restore should definitely work but for reasons unknown sometimes it fails for the reasons you describe. I already complained to Apple about it. It may have been fixed in Catalina.

Until they fix DU try the following Terminal command (example):

sudo asr restore --source / --target /Volumes/ShirtBox --erase

You will need to adapt that exact line for your specific installation (obviously... I took the liberty of assuming your backup disk's name). man asr for details.

The --erase flag seems to be necessary.

Here is an example of what you ought to see:

iMac11:~ john$ sudo asr restore --source / --target /Volumes/Untitled --erase
	Validating target...done
	Validating source...done
	Erase contents of /dev/disk4s2 (/Volumes/Untitled)? [ny]: y
	Repartitioning target device...done
	Replicating ....10....20....30....40....50....60....70....80....90....100
	Replicating ....10....20....30....40....50....60....70....80....90....100
	Restored target device is /dev/disk4s1.
	Remounting target volume...done
iMac11:~ john$ 

The first "Replicating" progresses from 0 to 100 rather quickly. The second (presumably "data") takes longer but the entire restore takes only a few minutes from start to finish. It results in an exact duplicate of your existing TM backup. TM won't know the difference.

One more thing, to preclude making the mistake of restoring the wrong disk.

Start by typing

sudo asr restore --source 

... followed by a space character.

Then, drag the source TM disk icon into the Terminal window.

Then type

--target 

... followed by another space character. Then, drag the target TM icon into the Terminal window.

Finally type

--erase 

... and press Return.

If something doesn't look right you have one last opportunity to bail out at the [ny] prompt.

Apologies for the remedial instructions but I don't want you to come back with the bad news that you inadvertently erased your one and only TM backup.

Time Machine drives need to be formatted as HFS+ Extended Journaled, case-insensitive (default). If Disk Utility reports that the Time Machine drive is Core Storage, then that is the wrong solution for Time Machine backups.

Here is the proper process to migrate an old Time Machine backup to a newer, larger external drive and continue the Time Machine backups on the new device.

Weird. Obviously you don't want to be without a backup, even for a short period of time, but I'd create a brand new backup on the new drive first. Then, consider reformatting the other one.

I understand you don't want to lose your entire backup history though, but that's one of several reasons I don't use anything less than three redundant backup devices. When a backup drive accumulates a couple years worth of backups I erase it and start a new set on that drive.

If you can't get Time Machine to work, I'd suggest doing the backups with Carbon Copy Cloner. It is not very expensive and as a bonus its backups are bootable. (Disclaimer: just a longtime happy CCC user).

https://macintouch.com/community/index.php?threads/backup.643/page-10#post-30288

When you go into Disk Utility go to the 'View Menu' and select 'Show All Devices' and select the top item above "ShirtBox' and erase that to clear all partitions on that drive