Migrate a Time Machine backup in terminal /Volumes/Original_Time_Machine is no longer being backed up to, but contains old backups that I want to keep.

/Volumes/New_Time_Machine_Archive is an empty partition

Both of these are on separate physical hard drives.

I was thinking of doing this in the terminal.
Connect both physical hard drives, then:

cd /Volumes/Original_Time_Machine
cp -rp * "/Volumes/New_Time_Machine_Archive"


Would I be able to restore from this if I copy this way? Or is there another way I should do it?
Apple has fixed Finder in macOS 10.13.4 to preserve hard links so my answer is to use Finder to migrate your TM backup. They did not, however, fix cp. It still does not preserve hard links and doesn't have an option to do so.

I know the OP requested a way of doing it from the terminal but all my efforts to do so in macOS 10.13.3, where I had given up on Finder because it did not then preserve hard links, failed.

The rsync command suggested by @n1000 failed after more than 12 hours with out of memory, even though I have 16GB of memory. This is a known problem fixed in rsync v3+. However v3 sets all the user and group ids to 99 and was painfully slow. There were long periods with no disk and no significant CPU activity. I've found rsync very slow on other platforms too.

cpio did not preserve ownership. 

pax got strange errors.

I came closest to success with tar:

cd /Volumes/My\ Passport
sudo bash
tar -cf - -T ~/TMFileList | (cd ../My\ Canvio;tar -xpvSf -)


where ~/TMFileList was created with

cd /Volumes/My\ Passport
sudo bash
find -d Backups.backupdb -print &gt; ~/TMFileList


[I tried a tar -c | tar -x thing first but that was copying some hard links as separate files for reasons related, I guessed, to the order it was seeing the files. Which led to me making the depth-first list.]

However tar was taking an impractically long time. I killed it after 3 days. Some of this was because it was extracting files multiple times. The majority of this was, like rsync, due to long periods of inactivity for reasons I could not determine. What it managed to complete had correct ownership and hard links were preserved.

So I strongly recommend using Finder. Just make sure you have macOS 10.13.4. Follow the Apple support instructions in the link at the end of @n1000's answer.