Not sure which vSphere and PowerCLI version you are running, but I checked again (vSphere 6.5 and PowerCLI 6.5R1), and the following works for me (with the VM powered on).
Get-HardDisk -VM MyVM -Name 'Hard Disk 1' | Move-HardDisk -Datastore MyDS
Blog: lucd.info Twitter: @LucD22 Co-author PowerCLI Reference