Switching Pi's

Hi, I have a Pi 4 all setup with DietPi that has been working greatly and now I bought a new Pi and want to start using it instead of the old one (was from a friend).

However, by unplugging the HDD and plugging into the new one, nothing seems to happen. I see just the green led blinking 3 times in a loop on the Pi and the system doesn’t seem to boot up (can’t even see it in the network).

What am I missing?

Is it a RPi4 as well? I guess you would need to boot from SD card one to update bootloader and probably activate USB mass storage boot.

3 times short flashes means a generic boot error