ssd cloned to hdd won't boot in USB

I'm upgrading my SSD on my Lenovo Yoga Pro 2 notebook.

In order to clone the original SSD, I wanted to first clone it to an external device, then boot the computer using the external device and clone it on to the new, now installed, SSD.

I managed to clone the SSD on an external HDD, but this HDD won't boot, blocking the process.

I am first getting a BIOS message saying no windows boot detected/no USB boot detected/no network boot detected. After this I get to manually choose where to boot from, I choose the USB, and it starts but crashes after a while.

Any idea what could be wrong? Is my strategy valid?