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I want to install Mojave onto an external drive, using a Mac that has Catalina as its current OS. The Mac is old enough to run Mojave.

A specific form of this question has been asked before, to which the answer is "create a USB installer disk".

I'm wondering if it might be possible without this workaround. Firstly, it requires the purchase of a USB stick. Catalina now requires just slightly more than 8GB, so I need a 16 GB stick at least. Admittedly, these are cheap and plentiful.

But I already have loads of 8GB sticks and the alternative to buying another bit of hardware is to sacrifice a 500 GB USB drive, which I already have, to become an installer medium.

However, creating an installer disk involves the createinstallmedia command line tool inside the MacOS installer app bundle. There are also several other tools, including: startosinstall, which yields the following info:

Usage: startosinstall

Arguments
--license, prints the user license agreement only.
--agreetolicense, agree to the license you printed with --license.
--rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
--pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
--installpackage, the path of a package (built with productbuild(1)) to install after the OS installation is complete; this option can be specified multiple times.
--eraseinstall, (Requires APFS) Erase all volumes and install to a new one. Optionally specify the name of the new volume with --newvolumename.
--newvolumename, the name of the volume to be created with --eraseinstall.
--preservecontainer, preserves other volumes in your APFS container when using --eraseinstall.
--usage, prints this message.

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It seems that there is a --volume flag to set the target drive, but it is only available when the OS has SIP disabled.

I tried disabling SIP and running startosinstall --volume <my external>, but the process just sat there, and Console was filled with messages about a binary not being signed correctly.

I suspect further investigation of this aspect of the tool may be required. I'm still hopeful that there's a way of doing this via the command line.

So I thought I'd put this information up here, even if it's just to get a confirmatory "No; duplicate", because the benefits to the community of getting a method would be large.

  • There is a GUI-type utility called DiskmakerX that will make a bootable USB installer for most versions of macOS. I've been using it for years and it works a treat, and no messing around with confusing command lines, just download the version for the version of macOS you want to install and away you go. – Steve Chambers Jun 1 at 15:20
  • @SteveChambers The issue is not the confusion of the command line, but seeking to cut out the step of the USB installer. – benwiggy Jun 1 at 15:31
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    I do not believe that is an option, especially when you are going back a version as the Mojave installer will not run under Catalina, meaning you need to boot under an earlier version of macOS to install a later version. In other words I don't believe this edge case is something that Apple has accounted for in building their OS installers, so you will have to create a bootable installer. – Steve Chambers Jun 1 at 15:39
  • It seems that the --volume flag is only an option when SIP is off, or when booting from Recovery, so it's possible that it can be done that way. I'll give it a try. – benwiggy Jun 1 at 15:43
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You don't need a USB stick, you can create a small partition on any existing drive & createinstallmedia to that, then select it with an alt-boot.

One thing you do not seem to be able to do is install a new OS to any other disk whilst there is a fully-bootable OS disk already present, it wants to 'update' the existing OS even if it's exactly the same OS.
You have to remove any current OS disk first, then it will ask you which to install to.

[Source of info - just spent the afternoon messing at this myself.]

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  • Normally, the current system is selected by default, but there's a "Show Other Volumes" button that then brings up others. I have installed or upgraded other disks than the startup several times (when the version numbers are even or greater). – benwiggy Jun 1 at 17:10
  • I never found one, & really, I was looking for one. Details, or picture if you've got one, so I can swear in private ;) – Tetsujin Jun 1 at 17:12
  • See this image. "Show All Disks". media.idownloadblog.com/wp-content/uploads/2020/01/… – benwiggy Jun 1 at 17:29
  • Nope, didn't get that, just got a forwards arrow. Only got to pick a disk once there was no viable OS connected. They really don't make this easy any more; I remember when you could install to a different drive right from the running OS itself. – Tetsujin Jun 1 at 17:45
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If you have extra space on an internal SSD then you can install the other OS on a new container/partition there. More reliable than external SSDs and their vulnerable USB/Thunderbolt cables.

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