As per this bug:
Debian Bug report logs – #852323
The Debian 9 installer (up to v9.5 at least) does not always configure GRUB to find the boot partition using UUID, but leaves it pointing to /dev/sdb or whatever. This can be a problem if you change disks in the system. In particular, if you install from a USB stick and then remove it when the system reboots after the install this can change the disk /dev id. The result is a unbootable system. This can be a bit fiddly to fix (although the first thing that’s always worth trying is to reverse the changes you made and see if it boots, e.g. plug the install USB stick back in to the same usb slot).
You can check this by looking at the /boot/grub/grub.cfg file. A quick check is:
grep "/boot/v" /boot/grub/grub.cfg
The fix is easy. Let the system reboot while leaving the install media in place (obviously make sure you don’t boot from the install media again!). Log in to the system and run (as root/using sudo)
update-grub
Compare the grub.cfg before and after. Then test by removing the install media and rebooting.