I gave up trying to install on external USB drive as I'd done in previous versions of macOS; I tried 11.1 & 11.2beta1 on 11,1 & 18,3 models. So with a Mac having external USB drive to hold full backup boot from USB memory stick setup by Big Sur "createinstallmedia" run Terminal "sudo ditto --nocache /Volumes/Macintosh\ HD/  /Volumes/<backup drive>" switch to Disk Utility and erase "Macintosh HD" switch to Big Sur installer & run it

My first time installing any version of macOS 11 was 11.1 release on 2013 MBP. Booted from "Install macOS Big Sur 11.1" flashdrive and chose to install on external USB drive. Progress bar started at around 28 minutes but has been displaying "Less than a minute remaining ..." for over 3 hours with the activity LED on external drive flashing the whole time. Is this another example of declining quality in macOS ?