I installed Stretch on my Thinkpad from a USB flash drive. Only after the installation, during the first boot did I notice that something was wrong. GRUB failed to load the kernel and after a short diagnosis I learned that for some reason, the Debian installer saw my USB thumb drive as /dev/sda and the SSD as /deb/sdb. This caused GRUB config to be wrong when I removed the flash drive after installation.
I did some testing and apparently, if I booted the Debian installer from my Thinkpad X220's only USB3 port[1], it would be recognized as /dev/sda, and if I used a USB2 port, it would be /dev/sdb. Doesn't the Debian Installer/GRUB not use UUID? What caused the problem here? I've never experienced such a fail before or since but it did cause some confusion and made me reinstall. [1] FWIW, apparently only (some of?) the i7 models of X220 have a USB3 port.