Open command prompt & open diskpart

DISKPART
list disk
select disk #
offiline disk 

Ctrl+C to leave diskpart

Enter PowerShell

powershell

Hyper-V 2012

Get-VMScsiController –VMName VMNAMEGOESHERE –ControllerNumber SCSICONTROLLERNUMBER | Add-VMHardDiskDrive –DiskNumber DISKNUMBER 

Hyper-V 2012 R2

Add-VMHardDiskDrive -VMName VMNAMEGOESHERE -ControllerType SCSI -DiskNumber DISKNUMBER  -Passthru