My MacBook Pro (running Big Sur) unexpectedly shut down this morning while my Mac OS Mojave virtual machine was active in Parallels. Now when I try to start up the Mojave virtual machine, the Apple icon appears and the progress bar completes, but then the system just stops... nothing else happens. When this happens to a physical computer, I boot up from an external CD or USB drive, run Disk Utility (Disk First Aid) on the internal drive, and the problem is solved 99 times out of 100. In some extreme circumstances, I've re-installed the OS from an external bootable drive or run in Recovery Mode on the internal drive to fix the problem. But NONE of those solutions work with my Mojave virtual machine! Even when I change the Boot Order (under Configuration > Hardware) so that my external drive loads instead of the internal drive, running Disk First Aid or trying to re-install the OS results in error messages that the Macintosh HD internal drive can't be umounted. But it shouldn't have ever been mounted in the first place since I booted from an external device, so I'm completely perplexed! Does anyone know how I can run Disk First Aid on a Mac virtual machine that can't finish loading the OS? I've even gone so far as to enable Select Boot Device on Startup (also under Configuration > Hardware) to select the "Mac OS X 10.14.6 Recovery" partition, but that runs into the same problem that the internal drive will not unmount for Disk Utility or reinstallation of the OS.