David Wright composed on 2017-10-07 15:01 (UTC-0500):
it might be worth pointing out that it has been reported here that
a USB3 stick inserted at boot can demote the internal disk to
/dev/sdb.
It need not be v3 of USB, and it's one of the prime reasons alternatives to
booting by device name were developed in the first place. Assignments are
typically BIOS dependent at least to some extent. The push isn't always to sdb
either. I've seen sda moved to sdf, sdg and sdh at times with some kernels over
time. Ultimately, device names come down to kernel version and whether the ATA
bus(es) are enumerated and initialized prior to USB(s) or not.