How do I delete files on a bootable osx Catalina disk from another bootup disk?
Is your Mac running a beta version of macOS? There is a known bug where the system crashes with lower amounts of space in Catalina. You should try resetting the NVRAM first. Then, you can also try a reset of the SMC for power related issues. To delete files off of the Internal Drive, boot up on the bootable drive and go to Finder and click on the Internal Drive. If FileVault was enabled, then macOS will prompt for the login password of one of the users on the problem instance of macOS. This will unlock and mount the drive. Then, you can use Finder and Terminal to navigate the files and delete.