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While I was starting my vm I got this error

Microsoft Emulated IDE Controller Error(Instance ID 83F863B-8DCA-9EDA-2CA8B33039B4): Failed to Power on with Error 'The process cannot access the file because it is being used by another process .'.

What's wrong with it?

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Do you have the VHD attached to another running VM? – Michael Hampton Mar 29 '14 at 19:36
up vote 4 down vote accepted

This error occurs when two virtual machines try to use the same DVD drive. To resolve this error, go to the virtual machine’s Media menu and un-capture the DVD drive. If the virtual machine needs access to the DVD drive then you will have to determine which virtual machine is presently using it and release the DVD drive from that virtual machine.

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It's also possible for Windows to lock a VHD file. I ran into this by double-clicking on a VHD, which causes recent version of Windows to open it up like a folder. However, it appears that the system then holds a lock on the file, which prevents Hyper-V from using it to start a VM. Rebooting fixed that problem.

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Rebooting is overly drastic. Just open Disk Management and un-mount the VHD. – longneck Aug 5 '14 at 19:09
That's even better – RedGreenCode Aug 5 '14 at 19:18

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