What boot order should I use to install Debian from USB on a ThinkPad?

For the installation, select the USB drive as the boot device. If the menu uses a generic label, look for something like USB HDD. After Debian is installed, remove the USB drive and boot from the internal drive where you installed it, often listed as ATA HDD0.

You may be able to use a one-time boot menu instead of changing the permanent boot order. The key varies by model, so check the startup screen or your ThinkPad's documentation.

If you're unsure which entry is the USB stick, compare the boot device list with the stick plugged in and unplugged. The entry that only appears when it's connected is the one to try.

Not seeing the USB drive at all? Try another USB port, then restart and check the boot menu again. Go through the available ports before assuming the boot order itself is the problem.