There would be a hard border, which would become relatively efficient over time but cause real disruption. The use of the Euro in Scotland would cause friction

There would be a hard border, which would become relatively efficient over time but cause real disruption. The use of the Euro in Scotland would cause friction for trade. More and more Scottish imports would come from Europe instead. England would continue to import from Scotland as it has no attractive alternative. Realistically there are only two paths forward for England anyway: either the EU will fail completely, or England will eventually be forced to rejoin. So the border will only be temporary.