How do I install Mojave on a bootable external USB3 when currently running Catalina?

[paulwgraber]

Starting a new thread on this. I have been trying to create a bootable USB3 Mojave thumb drive so I can run my 32 bit apps on it. I am currently running Catalina on my main system.
I have googled and followed several instructions on how to do this. However I have been unsuccessful. I even was using two usb drives to try and leave Catalina out of it completely.
Came close but upon my install Mojave bootup it freezes and shows my my mouse and keyboard icon as if they are not connected. I m going nuts here trying to figure it out and all tuturials pretty much say the same thing but have come up short. Please help.
I have a USB3 256G that I want to put Mojave and my apps on which is plenty of storage.
Thx

 

[DeltaMac]

If your keyboard & mouse are wireless, and you need them to finish the setup, you would need to swap out to a USB wired mouse and keyboard. It's one of the reasons I keep a pair for spares, just for such occasions.
(I would suggest that running a full system on a USB thumb drive is probably not your best choice. A true SSD (in a USB enclosure) will give you a better experience. Thumb drive is good for installing the system, but for a running system that you can actually use for "things", will be a better with an SSD.

 

[paulwgraber]

Well I am using a ssd to actually run it but had to use another thumb drive to boot with so I could install it onto the SSD. I can't do this while in my Mac because it has Catalina on it.

 

[DeltaMac]

eh?
If you have a bootable Mojave installer on that thumb drive - then it doesn't matter that you might have Catalina installed on the internal drive.
Just boot to that Mojave installer, and choose the external SSD as the destination for the install.

If you DON'T have a bootable installer on your thumb drive, follow the steps to make one HERE,

You can also use one of the third-party apps that easily make a bootable installer - such as DiskMakerX.

Either of those solutions will work for you to make a bootable installer.

 

[Fishrrman]

Do you already have the Mojave installer? (the whole installer which is about 6gb, not just the "stub")
If so, I'd do what DeltaMac suggests -- that is, use DiskMaker X to create the bootable USB flash drive.

Then, power down, ALL THE WAY OFF.

Put the bootable flash drive in one USB port.
Put the USB3 SSD in another USB port.

Press the power-on button and IMMEDIATELY hold down the option key and KEEP HOLDING IT DOWN until the startup manager appears.
Then select the USB installer and hit return.

Can you get booted to the installer this way?

If so, do this next.
a. DO NOT run the OS installer yet. Quit it if it opens.
b. You should now be at the utilities menu. Open Disk Utility
c. Go to the "view" menu and choose "Show ALL devices".
d. Locate the SSD in the list.
e. Now you want to ERASE the SSD to APFS with GUID partition format
f. Quit disk utility and re-open the OS installer
g. See if it will install onto the SSD now.

 

[bogdanw]

Would it be possible to use startosinstall?