Limitations on Changed Block Tracking

Changed block tracking does not work if the virtual hardware version is 6 or earlier, in physical compatibility RDM mode, or when the virtual disk is attached to a shared virtual SCSI bus. ESXi 3.5 supported only up to virtual hardware version 4.

Changed block tracking can be enabled on virtual machines that have disks like these, but when queried for their change ID, these disks always return an empty string. So if you have a virtual machine with a regular system disk and a pass-through RDM as a data disk, you can track changes only on the system disk.