I used Recovery OS Disk Utility to mount (unlock) a Core Storage encrypted volume, then Terminal to attempt HFS-to-APFS (no conversion). I can't recall whether the (Command-Q) quit from Terminal was preceded by Command-W closure of the window. I probably did exit before the quit. When I first posted this topic, I recalled the issue as: after using Terminal, macOS Utilities failed to reappear. Symptom: a grey screen persisted for a few minutes. I chose to force off the power. Postscript Whilst attempting to reproduce the symptom, I realised the more likely steps to reproduce: with Recovery OS Terminal, use diskutil(8) to unmount any volume that might be used for startup quit Terminal open Startup Disk quit Startup Disk Environment Booted from an external hard disk drive, MacBookPro8,2 limited to USB 2.0.