Too bad there is has not been any response because booting from an USB drive attached to one of the router’s exterior USB ports works with no issue.
and would have saved me from wasting funds on
Had three USB drives attached to the router (one via interior mPCIe adapter and two on the exterior ports) at boot time.
Perhaps the online doc stipulation is owed to the boot env setting root=sdX where the X could vary with multiple USB drives attached. However, u-boot provides for a more specific designation with root=PARTUUID= and thus does not get mixed up.
The caveat is that schnapps provides with ROOT_DEV only the sdX designation and one need to be aware of such when utilizing schnapps on such USB drive.