Can't reinstall Windows 10

Hello,

I just connected the old SSD from my laptop to my PC. It had Windows on it, but I wanted to reinstall it and then use it on my main drive on here. I created a bootable USB, booted from it and deleted the partitions on that drive then selected it's unallocated space to install Windows on. It was going fine and said it was installing etc, then it said it had to restart. I let it, but it booted into my main SSD so then I restarted, opened the boot menu and booted from the laptop SSD. Now, it comes up with an underscore flashing in the top left corner and nothing else... How can I fix this?

Thanks.

EDIT: I tried doing the same thing but booting into the USB when it restarted, but it just brought me back to the start.

EDIT2: I seem to have fixed it by going into the boot menu and then booting from the laptop SSD when it restarts, rather than letting it boot into my other one.