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I have a 2012 Mac Pro 5,1, running El Capitan. I wish to make it a dual-boot system, El Capitan and Mojave. I understand that I must install High Sierra preliminary to Mojave, and must install a "metal"-capable graphics card (I've purchased, but not yet installed, a Sapphire PULSE Radeon RX 580).

I have my El Capitan system on a 240GB SSD; user directories are on a separate HDD. I've created a 2nd partition on the SSD and done a clean install of High Sierra there. The installer made it an APFS volume, so when I am booted into El Capitan, I cannot select High Sierra as my Startup Disk (El Capitan cannot "see" the APFS volume in System Prefs). I can select High Sierra using the Startup Manager (⌥ Option during startup). But my understanding is that once I install the new GPU (prior to upgrading to Mojave) that the Startup Manager will not work (no boot screen) Thus, once I boot into El Capitan, there is no way to then boot into High Sierra.

Is it possible to resolve this? I've read that it's possible to install High Sierra on an HFS+ partition, but not Mojave. Perhaps I can live with High Sierra and Mojave as my two boot options instead, if I can get my older apps to work on High Sierra? But even then, without a boot screen, it's impossible to get to recovery disks etc (what happens if the boot drive fails?). Is the only option to obtain a GPU that supports the boot screen? Apparently macvidcards.com sell such cards and could modify mine, but they appear very sketchy, from searches I've done.

EDIT: This post may partially be a wild-goose chase - my apologies. I've determined I can probably run all my apps under High Sierra (and possibly under Mojave) - I was mistaken/confused originally - so it's looking like I may not need to boot ElCap anymore. But there may be other users who do need to, so I still think it's a discussion worth having. However, the boot-screen issue remains - in particular being able to boot from recovery partitions or external media.

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  • Suggestion: put Mojave on a separate drive, then swap to whichever you wish to boot to. I'm not sure how you'll fly blind with a non-flashed GPU to try swap over otherwise. I have one Mac Pro with a non-flashed card but if it fails I have flashed ones I can swap in for troubleshooting. – Tetsujin Apr 30 '20 at 18:26
  • I still don't see how I'd swap to the Mojave drive, from ElCapitan, without a boot screen - since the Mojave APFS volume can't be seen when logged into ElCap. IOW, I don't see how putting Mojave on a separate drive helps me; what am I missing ? – RustyShackleford Apr 30 '20 at 18:38
  • @RustyShackleford Use Startup Disk preferences. Select the disk with macOS Mojave and enter the password to decrypt the disk if FileVault was on. Then select restart to startup on macOS Mojave. – Todd Apr 30 '20 at 19:18
  • @Todd The problem is that if I'm in ElCapitan, that SystemPrefs->StartupDisk cannot "see" Mojave (it can't even see HighSierra now). Are you guys trying to tell me that if I put HighSierra/Mojave on a separate drive (instead of on a separate partition as it is now), that somehow I'll be able to select it as StartupDisk ? – RustyShackleford Apr 30 '20 at 20:33
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    Rusty - my idea was literally to take one drive out & put another drive in, something which takes a whole 10 seconds on a Mac Pro… but I'd guess David's rEFInd suggestion would be easier ;) – Tetsujin May 1 '20 at 8:31
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One possible solution is to install rEFInd Boot Manager. This is free software which can be configured many different ways. Below is a suggested configuration.

By default, rEFInd installs in the EFI partition. You can then configure rEFInd to either boot El Capitan or Mojave. The default operating system to boot can be set directly from the operating system. There is simple AppleScript application you can download which will do this. As example of the application is shown below.

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  • Looks intriguing, gonna require a little digging to figure it all out, so not upvoting yet. Thanks – RustyShackleford May 2 '20 at 3:46
  • With this, I still need to be booted up on one OS to change to another, right ? So it doesn't really get around the boot-screen issue ? – RustyShackleford May 2 '20 at 17:35
  • The idea is to use the AppleScript application instead of the Startup pane of System Preferences to choose the default operating system to boot. You would have to install the AppleScript application in both operating systems. The application reads the choices from a rEFInd configuration file stored in the EFI partition. Any change in choices is written back to this file. When you boot/reboot your Mac, rEFInd reads this configuration file and silently instructions the firmware to boot your choice of operating systems. – David Anderson May 2 '20 at 18:38
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    The EFI partition is normally there and normally invisible to the Finder and Disk Utility. Operating systems such as Windows and Linux use the EFI partition to store boot code. MacOS does not use the EFI partition to store boot code, but requires an EFI partition for reasons which have never been fully spelled out. The rEFInd boot manager normally resides entirely in an EFI partition, although this is not a requirement when installing on intel Mac computers. The EFI partition is FAT32 formatted and is usually 200 to 400 MB in size. – David Anderson May 6 '20 at 4:18
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    The rEFInd Boot Manager can be configured to include the recovery partitions. However, this is a binary flag. In other words, you can display either all recovery partitions or no recovery partitions. You can configure rEFInd to scan and display external bootable media. However, some external media may not be found when rEFInd scans. Therefore, a manual refresh of rEFInd may be required before the external media is displayed. This can be avoided, if you manually configure rEFInd to display known external media. – David Anderson May 23 '20 at 22:35

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