While working on an Exchange 2013 CU6 upgrade, the upgrade got stuck at 76% on the "Mailbox role: Mailbox service" step. It had been stuck there for about 45 minutes.
I looked at the ExchangeSetup.log file in C:\ExchangeSetupLogs directory and saw the last update was about 45 minutes ago and was:
[09/20/2014 04:08:20.0184]  Starting: net.exe with arguments: share address="E:\Program Files\Microsoft\Exchange Server\V15\Mailbox\address" /grant:"NT AUTHORITY\Authenticated Users,read"
It seems to have stopped right after trying to create the share for the address book. I opened Computer Management to see if the share was there and had proper permissions and I got the following error:
In the Services control panel and saw that indeed the Server service was stopped. After starting it and waiting another 5 minutes the progress was not increasing still.
Then in Task Manager there where the net.exe and net1.exe processes where running. After force stopping those processes the percentage of the task starting going up.
I had to reboot this server before I started the update because it was pending a reboot, I suspect that the Server service didn't start upon reboot.