I am using the latest High Sierra operating system 10.13.4 on an iMac, which I had been backing up just fine to an external USB drive connected by USB cable to the iMac, but inadvertently, a day ago, I got this backup to work with a new hard drive rather than the iMac, and now I need to get it to work again with the two years of backups on that USB drive. Time Machine does not recognize the backups on this drive as being associated with the iMac: with the option key I can select the USB drive as having the backups but these cannot be read or incremented by Time Machine. I want to use tmutil to restore that association.

What I know is that I do have the USB backup drive connected and usable, with many Gb of space (but not the Tb needed for a completely new backup) and I can see everything on it with the finder and with terminal. But somehow the command for tmutil is not working. Whatever I try, I just get the response

"A local volume mount point and a snapshot volume path are required."

Here are examples I have been trying, for backups of the entire iMac, which is called the usual “Macintosh HD”, to a USB drive called "4TB Seagate"

sudo tmutil associatedisk [-a] / /Volumes/4TB\ Seagate/Backups.backupdb/Bruce’s\ iMac/2018-05-11-061933/Macintosh\ HD

sudo tmutil associatedisk [-a] / “/Volumes/4TB\ Seagate/Backups.backupdb/Bruce’s\ iMac/Latest/Macintosh\ HD”

sudo tmutil associatedisk [-a] "/Volumes/Macintosh HD" “/Volumes/4TB Seagate/Backups.backupdb/Bruce’s iMac/2018-05-11-061933/Macintosh HD”

I cannot get the command tmutil inherit backup to work either

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