Synthetic SCSI Controller failed to Power on with Error access is denied

I was working on setting up a VM for my server farm and mis-configured one of the vhdx drives. I ended up having to delete that drive and recreate it in Hyper-V manager. When I did though, I received an error stating that I could not start the virtual machine:

An error occurred while attempting to start the selected virtual machine(s).‘VMName’ failed to start. (Virtual machine ID ‘SomeID’)‘VMName’ Microsoft Emulated IDE Controller (Instance ID ‘SomeID’): Failed to Power on with Error ‘General access denied error’ (0x80070005). (Virtual machine ID ‘SomeID’)‘VMName’: IDE/ATAPI Account does not have sufficient privilege to open attachment ‘C:\Users\Public\Documents\Hyper-V\Virtual hard disks\DiskName.vhdx’. Error: ‘General access denied error’ (0x80070005). (Virtual machine ID ‘SomeID’)

‘VMName’:  Account does not have sufficient privilege to open attachment ‘V:\Hyper-V\Virtual hard disks\DiskName.vhdx’. Error: ‘General access denied error’ (0x80070005). (Virtual machine ID ‘SomeID’)

Causes

Each virtual machine is started using a virtual machine account. The virtual machine account needs read and write access to the .vhd/.vhdx file, but if the file has just been copied from somewhere then it most likely lacks the necessary file permissions.
That happened in my case because I had just created the vhdx drive and did not create it from the VM itself. I just attached it to the VM. So, when I booted the VM, it gave the error.

Remediation

There are a few ways that you could remediate the issue. The simplest way, if it is a new VM, is to remove the drive in the VM settings and then re-create it from scratch. That is what fixed it for me.
Another way is to add the VM GUID to the permissions so that it can access the vhdx file properly:

• If you don’t already have the Hyper-V Manager error dialog open (“An error occurred while attempting to start the selected virtual machine(s) …”) then try to start the virtual machine now. You need the error open.
• Click “See details”. This will show additional details, and will look something like:

‘PC-Name’ failed to start. (Virtual machine ID B9C4F7D4-0009-4BE2-90FB-9D60B1A06BDD) ‘PC-Name’ Microsoft Emulated IDE Controller (Instance ID XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX): Failed to Power on with Error ‘General access denied error’ (0x80070005). (Virtual machine ID B9C4F7D4-0009-4BE2-90FB-9D60B1A06BDD)‘PC-Name’: IDE/ATAPI Account does not have sufficient privilege to open attachment ‘E:\Hyper-V\PC-Name\Virtual Hard Disks\MyVHD.vhdx’. Error: ‘General access denied error’ (0x80070005). (Virtual machine ID B9C4F7D4-0009-4BE2-90FB-9D60B1A06BDD)‘PC-Name’: Hyper-V Virtual Machine Management service Account does not have sufficient privolege to open attachment ‘E:\Hyper-V\PC-Name\Virtual Hard Disks\MyVHD.vhdx’. Error: ‘General access denied error’ (0x80070005). (Virtual machine ID B9C4F7D4-0009-4BE2-90FB-9D60B1A06BDD)

Where PC-Name will be the name of your virtual PC. The long sequence of letters and numbers (in my case above “B9C4F7D4-0009-4BE2-90FB-9D60B1A06BDD”) is the Virtual Machine ID. This number is significant and you need it to fix the problem.