I am in the process of updating our 3 Mac Pros to Mountain Lion, in the office.
One that I updated a couple of weeks okay got stuck on a grey screen while booted in Windows 7 on BootCamp. I think it was downloading updates from Apple for BootCamp 3 at the time. As it was like this over at least an hour, I had to perform a hard shut down.
I did performed a Recovery Mode and tried checking the hard drive and repairing permissions with Disk Utility but still got the grey screen on reboot.
I then tried a Safe Boot (holding down shift on reboot) and managed to get back into Mac OS X.
Realising that BootCamp 3 wasn't compatible with Mountain Lion, I asked our IT dept to update the Mac to BootCamp 4 yesterday and all seems well again now. Although, I haven't tried this morning.
I booted my own Mac this morning and got the stuck grey screen during start up. Now, this cannot be the same issue as above, as I don't have BootCamp on my Mac, but use Fusion VM Ware with Windows 7 instead (version 5).
After doing Recovery Mode and checking the hard drive and repairing permissions, I tried a reboot. Still stuck on grey screen.
Rebooted in Safe Mode and got Mac OS X back, so thought it must be a start up item. I deleted ALL log in items listed under my User preferences and rebooted. Still stuck on grey screen.
I then reset my PRAM, and rebooted. Still stuck on grey screen.
I have verified my preference files using OnyX, and they all appear to be in order.
The only other thing I can think of is that I updated Flash before shutting down last night.
I'd be grateful for any other suggestions, as I'm not sure I can work in Safe Mode for the rest of the day.