Upgrading to Mojave via External Hard Drive

Chavista wrote:

I want to upgrade my MacBook Pro from Sierra to Mojave. Rather than do a typical install, I want to clear the junk that has accumulated on my hard drive by wiping everything away and installing Mojave on a blank slate.

If I understand correctly, I can boot up from an external hard drive that I've cloned my internal hard drive to by holding down the option key while I restart my laptop. If this is true, then I assume I can work on my projects in my external hard drive while I'm installing Mojave on my internal drive.

How to set up and use an external Mac startup disk - Apple Support

https://support.apple.com/en-us/HT202796

I assume I can work on my projects in my external hard drive while I'm installing Mojave on my internal drive.

No, the installer will take over and you can't do anything else.

If so, is there a way to tell it to install Mojave on my INTERNAL hard drive not the external drive I'm currently using?

When you run the installer, it asks where you want to install.

Or is there a better way of doing it?

Not to achieve what you want which is not possible.

The clone is likely the best choice, though.

P.S. After I upgrade to Mojave, am I going to be able to access my external hard drives while they're still in Sierra mode?

You should be able to boot into a drive with Sierra on it. That drive would not be able to see the Mojave drives as they will be APFS and Sierra cannot read an APFS volume. You can see the Sierra drive from booting into Mojave.

To erase the drive, you will need to use Disk Utility, first.

If you clone the external and boot from that, you could erase the internal from there. It would also help confirm that the clone works before erasing.

When you go to erase the drive in Disk Utility, select the drive device (outdented and above Macintosh HD).