Time Machine disk copy stuck on 5 seconds remaining

I have twice tried to copy a 1TB Time Machine disk to a new 5TB disk using the Restore command in Disk Utility. Both times, the copy seems to be completed (the blue progress bar is full) but the Copy window does not close, instead becoming stuck with the message {"Copying 0 items to (disk name)" 1.82 TB of 1.82 TB - About 5 seconds.}

If I try to eject either disk or close Disk Utility, I get a warning that a copy is still in progress.

The first time, after ten hours of the 5-second warning, I forced the Mac to shut down but upon re-start, the copied disk was unreadable and would not mount. What's worse is that the source disk now reported a disk problem and suggested I repair it, but when I tried, the repair failed, multiple times, and I could not even mount the source disk.

I left the disk connected for several hours and eventually was able to mount it but it no longer allows me to recover recent data via Time Machine! Only the oldest state is visible. This is disastrous as I have to recover files that were only present in a recent backup.

Yesterday I started a new Restore cycle, but after another all-night copy, the window has again been stuck for over 5 hours at the 5-second warning.

I need to salvage the contents of the original Time Machine disk and would love to know why the process keeps apparently stalling at the 5-second mark.

Any help would be greatly appreciated.

Meanwhile, I will leave the disk connected and Disk Utility running for several days and see if it miraculously fixes itself.