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On a Macbook (early 2009) w/ 4gbs of ram instead of 2, using an external hard drive at the startup ios disk utility, is it possible to (forgive my terminology) open a disk image or something that has a Mojave on it? currenty at 10.10.5
MacBook, OS X 10.10
If the Mac supports macOS 10.14 Mojave, you can download and install it via a bootable USB flash drive to an external drive and Option-boot to it. If you are still on an earlier macOS, you might skip the USB flash drive step. Backup before doing anything!
How to download macOS - Apple Support
Create a bootable installer for macOS - Apple Support
If the Mac supports macOS 10.14 Mojave, you can download and install it via a bootable USB flash drive to an external drive and Option-boot to it. If you are still on an earlier macOS, you might skip the USB flash drive step. Backup before doing anything!
How to download macOS - Apple Support
Create a bootable installer for macOS - Apple Support
OS X El Capitan - Technical Specifications
So based upon the provided information the answer is NO
Updating iOS at startup using separate hard drive
