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Original post by: Ron Jankowski ,

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Dan, that is exactly the answer I needed. I didn’t know how Apple set up the two ports with a fusion drive but it looks like both ports are independent since I ordered my system with only a 2TB SSD NVME card.  Thanks again.

Ron

The following bit of information has nothing to do with SSDs but it is important if it saves any other people from headaches when  upgrading to a new system with the APFS file structure in High Sierra or Mojave. ” DO NOT BOOT FROM AN EXTERNAL MAC OS DRIVE WITH ANY OS that was released before HIGH SIERRA on a MAC withan APFS file structure..  I tried it to check a Sierra drive from my old unit to see if it still booted. It booted fine but I had to completely rebuild my original Mohave partition on my brand new system after the boot.  It seems like the external drive tried to communicate with the internal drive and the file structure was different. That caused either something to be written to or something was erased in the new system’s boot area.  The Mojave drive could no longer mount. Disk utility showed the partition only as “APFS”(No drive Name) and it was grayed out. I didn’t see any warnings anywhere in current documentation (I could have easily missed it if there was a warning) so I thought an external boot wouldn’t write anything and that it would probably not see the newer OS.  Boy was I wrong  - after many hours re-establishing Mojave and all the transferred apps.

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