It is recommended that all users update the firmware on Pi 400 if they are unable to boot from usb. It might be easiest to do this using Pi OS first as there is good documentation for this on the Raspberry pi site. There are some issues where the Pi 400 won’t boot on SSD without using a kernel parameter specifying the VID and PID of the storage device.
Add the following line at the beginning of the
[PID] parts with the USB Vendor and Product ID of your SATA to USB 3.0 adapter.
Use this command to find the VID and PID of the device.
VID is the first number and PID is the last one