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@tw67550 wrote:

In my case I‘am running  the VM with Fusion Player 12.1 on a Big Sur 11.2.1 host updating the VM from 11.3 beta1 to beta2 . Could be a space problem but I can‘t check. The vm is in Loop...

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If you have enough disk space on the host, you might be able to check the free space in the misbehaving VM by creating another macOS Big Sur VM then adding the misbehaving VM's virtual disk as a second disk on the new VM. (I haven't tried this technique yet but I've seen it mentioned before.)

Another option might be if you can boot into the recovery volume in the misbehaving VM, its Disk Utility (or corresponding Terminal commands) should be able to reveal the state of the disk.