Can I extend a virtual hard disk file in use by a Windows Server 2012 Hyper-V virtual machine?

A: It's not possible to extend an in-use virtual hard disk (VHD) or VHDX file in Windows Server 2012 Hyper-V. Several alternative actions exist that minimize the potential impact of not being able to extend a VHD on a running virtual machine (VM).

There are other more creative options available that I wouldn't recommend but might work. One example would be to add more VHDs to a VM on the SCSI bus, then within the VM create a spanned disk using the new disk.

If the guest is Windows Server 2012, you could create a Storage Space within the VM, as it's easy to just add additional disks to an existing Storage Space extending its size. Do NOT try and create a Storage Space in a VM using passthrough disks, as when the host reboots, the host would see the disks have a Storage Pool and try to claim it.

I would also point out Storage Spaces is designed to be used on the host with physical disks and is really not supported in a VM, but I wanted to at least include it here to cover all bases. Another option would be to use disk passthrough to a LUN on a SAN that supported extension.  However, using a passthrough disk means not using a VHD and therefore you lose the benefits of VHD for the VM such as snapshot, backup, VM migration and the use of Hyper-V Replica.