Same situation. Installation stucks at "less than 1 minute left" since 2 hous. Before updating the old system I would like to test my apps unter Big Sure on an USB stick. I switched off my mac pulled out the install USB stick and tried to reboot from the unfinished installation. It started to boot from the stick, but repeated the installation (28 Minutes left). It ended again in "less than 1 Minute left". Any suggestions?