The number of disks on the virtual machines must be equal to the number of disks on the physical machines.
The size of the disk on the virtual machine must be equal or greater than the size of the disk on the physical machine.
When configuring virtual hard disks, you must make sure that the virtual disks are in the same sequence as the disk numbers displayed in disk manager on the physical machine.
The boot disk should be same as the original one. You may need to configure the boot sequence of hard disks in BIOS setup of virtual machine as shown in the following illustration:
