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I've installed Mojave fresh, direct using an Apple-downloaded installer 4x now on 2 brand new, freshly formatted Seagate 2TB drives as well as a partition on another reliable non-Seagate drive. Drives were formatted Mac OS Extended journaled and GUID and connected directly to usb on specced-out Mac Mini 2012 running Mavericks. In all cases install was smooth and ended with booting to fully operational Mojave installs on external drives. All good so far.

Restart as-is ie back to fresh-installed Mojave on external USB drive and all good, boots to external Mojave with Mac Mini on-board OS and files also visible as expected in Finder.

Reboot to the original on-board OS (Mavericks) either by selecting in prefs>startup disk or by shutdown and disconnect/reconnect external drive and the new Mojave partition doesn't show as an OS or on the desktop or anywhere. In Disk Utility it's now greyed out and renamed to disk1s2. A 2nd non-OS partition on the same external drive shows up as normal, just the Mojave partition has gone AWOL and doesn't appear in prefs>startup disc either.

I tried using Disk Utility to check/repair after this all happened. 'root' external disk shows no problem, non-OS partition shows no problem, Mojave partition is greyed out and has random generic name disk1s2.

User passwords and computer name are all different.

The output from Maverick's diskutil list disk1 command for an external drive is shown below.

/dev/disk1
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *2.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2: 7C3457EF-0000-11AA-AA11-00306543ECAC               1.0 TB     disk1s2
   3:                  Apple_HFS spare B                 999.5 GB   disk1s3 
  • I presume the Mojave drive is APFS. Mavericks cannot read APFS, it doesn't even know what it is. I'd suggest trying to at least get a Mojave recovery partition on the internal drive, which would then possibly give you the opportunity to choose the external from Opt/boot. – Tetsujin Nov 17 '18 at 8:14
  • All 3 external drives I tried work fine other than w/ the Mojave install. In fact the Mojave install works fine until reboot. Do you mean that the Mojave install is AFPS and is unreadable by Mavericks? – bruceleeperry Nov 17 '18 at 8:29
  • For the external drive this is what I get: /dev/disk1 #: TYPE NAME SIZE IDENTIFIER 0: GUID_partition_scheme *2.0 TB disk1 1: EFI EFI 209.7 MB disk1s1 2: 7C3457EF-0000-11AA-AA11-00306543ECAC 1.0 TB disk1s2 3: Apple_HFS spare B 999.5 GB disk1s3 – bruceleeperry Nov 17 '18 at 10:01
  • in that result I presume 2: is the AFPS Mojave pertition. 3: is the empty partition that still appears normally – bruceleeperry Nov 17 '18 at 10:03
  • The value 7C3457EF-0000-11AA-AA11-00306543ECAC means the Mojave installation partition on the external drive was converted to APFS. Mavericks does not recognize APFS so the output from diskutil shows the GUID for an APFS partition. You can get a list of GUIDs for GPT partitions from Wikipedia. – David Anderson Nov 17 '18 at 10:49
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The simplest way to boot back to Mojave would be to reboot the Mac and immediately hold down the option key. This will invoke the Startup Manager. Select the Mojave startup disk. Next, click on the arrow below the icon to boot Mojave. If you wish to make Mojave the default, then hold down the control key before clicking on the arrow.

Apple a recently added new instructions for using the Startup Manager. These instructions can be found on the Apple website: How to select a different startup disk.

While it is technically possible to select Mojave directly from Mavericks, this would involve installing a third party boot manager such as rEFInd. Unless you intend to change operating systems often, such an installation would probably not be worth the trouble to pursue.

  • Thanks for the rEFIND info etc. I had tried Startup manager. It seems from Tetsujin's comment that Mavericks doesn't recognize the AFPS format that a Mojave install applies, rendering the disc invisible during reboot....I'm learning – bruceleeperry Nov 17 '18 at 10:08
  • @bruceleeperry: Are you saying Mojave does not appear in the Startup Manager? I ask this because I have Mojave installed only on an external drive and have no problems booting Mojave from the Startup Manager on my 2013 iMac. – David Anderson Nov 17 '18 at 10:36
  • @bruceleeperry: Are you saying Mojave does not appear in the Startup Manager? I ask this because I have Mojave installed only on an external drive and have no problems booting Mojave from the Startup Manager on my 2013 iMac. To be clear, you do realize the Startup Manager is not the same as the Startup Disk pane in Maverick's System Preferences application. – David Anderson Nov 17 '18 at 11:00
  • I can't actually access the Startup Manager - I've tried and have a wired keyboard connected directly to the Mac Mini, no hub etc. I also tried updating the Recovery OS as per forums.macrumors.com/threads/… but get hdiutil: eject failed - No such file or directory – bruceleeperry Nov 17 '18 at 13:19
  • when you say you boot from a 2013 iMac, what OS is that iMac running? Something that's able to see AFPS? – bruceleeperry Nov 17 '18 at 13:20

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