Q. Why do I receive an error message that the Virtual Hard Disk (VHD) file could not be attached to the IDE controller when I start a Microsoft Hyper-V virtual machine (VM)?

A. I recently had this exact problem when I moved my lab’s virtual hard disks to an external RAID 0 7200rpm drive to get better performance during demonstrations. I received a General access denied error when I tried to attach the VHD to the IDE controller, as Figure 1 shows.

There are a number of possible causes. First, check the VHD file security. You need to ensure that you're logged on as a local Administrator, and that the NTFS permissions on the VHD are set so that the System account and the local Administrators account have Full Control. The Users group should also have Read and Execute permissions.

If the virtual hard disks are on a file server, you need to ensure that they're connected via a Universal Naming Convention (UNC) path (you can't use a mapped drive or mount point), and that the Hyper-V computer account (domain\computername$) has Full Control VHD-file permissions and it's on the share containing the VHD.

The solution that worked for me was to remove the device from the VM, then add it again using the following procedure:

1. Select Settings for the VM experiencing the problem.
2. In the Hardware menu, select the problematic VHD hard drive and click the Remove button, as Figure 2 shows.
3. Select the IDE controller, then the hard drive virtual-disk file, and click Add, as Figure 3 shows.

In the new device's Properties' menu, click the Browse button, select your existing VHD file, click Open, then click OK in the main Properties dialog box. Your VM should now start.