You can only resize the virtual hard disk of a running virtual machine if:

  1. It is a .VHDX file
  2. It is connected to a SCSI controller

These restrictions mean that people often ask me if you can resize the boot disk of a virtual machine.  And the answer is: Yes, if it is a Generation 2 virtual machine.  Generation 2 virtual machines only have SCSI controllers, and boot off of the SCSI controller.

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Meanwhile, Windows has supported shrinking and expanding the boot partition since Windows 7 / Windows Server 2008 R2.  So you can easily resize the boot partition after you have expanded the virtual hard disk.

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Cheers,
Ben