Excuse my English but i’m French…
I had a RPi 3 B+ that i have migrated to RPI 4.
With my RPI3 B + I have flashed my SSD (not HDD) with a backup of my SD card made by the DD command made under linux of my SD card.
Then i extend the partition with gparted.
With RPi 3 B + there is nothings to do, to boot RPI3 B+ with HDD (with correct power supply) or SSD.
With my RPI 4 i have installed Raspbian then yunohost with curl method. But before to boot on SSD or HDD there is a specific manipulation (flashing eeprom).
Finally in post install i have restore back up from my previous installation on RPI 3 B+.
You can see here too: