Could it have anything to do with the OS? I am running Sierra after all...It is. If the tip in the previous post doesn't do it, please do the following: Put the old mini in Target Disk Mode and connect it to the new mini. On the new mini, open System Profiler and post a screenshot of its Thunderbolt section. This allows checking if the new mini "sees" the old one. Then, post a screenshot of Disk Utility on the new mini.