I received an update message for security update 2019-006 10.13.6 (macOS High Sierra) on both my iMac late 2012 and my MacBook Mid 2010. Both are running OS 10.13.6: The update showed on the iMac first so I downloaded to the iMac first but on
the restart received a message the "macOS could not be installed on this computer couldn't mount disc, quit the installer and restart your computer and try again"...I did as instructed, same message. I then went to the MacBook downloaded and the update installed without any problem. Thinking there was a problem with my connection before, I tried the iMac again. Again same message as first two attempts.

Anything I can check on the iMac that the installer could be not seeing the OS on restart?

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