I suggest installing Raspbian to the SDCard, transferring the
rootfs to the HDD and then install using emonScripts but without the initial bit re preparing the SD card - i.e. just do the curl and run the script.
Also I strongly suggest not booting from the HDD, but continue to use the SDCard for boot as long term it is more reliable.
An HDD is just as susceptible to power cuts as they are often accompanied by slight power surges. As a minimum have a surge protector, but I’d invest in a small UPS so the power down can be controlled.
There are a couple of bit on the Raspbian forum on moving to HDD
Use the PARTUUID not the device name.