1) Upgraded to Mojave
2) Found an old 1TB external HD. Wiped and reformatted. For some reason I get this error message every time I try to install Mojave on it. It gets all the way through and just before it says it has to reboot, I get this. Have tried several times and rebooted in between and always get this.



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I would first try opening disk utility, and with no other drives connected, reformat that target drive. You can use MacOS extended Journaled. Then try the install once again.



You can also try throwing that installer away (located in the applications folder) and redownloading it.

Also, make sure the operating system you are currently on is equal to the version you are trying to install.

Lastly, if neither of those work, you an create an installer on a separate external drive like a flash drive (you'd need at least an 8 gig thumb drive). There is a built in terminal command that allows you to create an installer. Then you'd boot to the external flash drive to install on the other drive. More here: How to create a bootable installer for macOS - Apple Support