Last I encountered some errors while doing a storage VMotion for a couple of VMs during a migration from ESX 3.5 to vSphere 4.1.
When looking in the logs for the CBTMotion errors I found following messages.
CBTMotion: scsi0:1: Clone: failed to start block tracking session on source disk: Change tracking is not active for this disk.
CBTMotion: scsi0:1: failed to clone disk from /vmfs/volumes/694286b5-b7bf1d73/VM01/VM01_1.vmdk to /vmfs/volumes/2b0e242d-40129/VM01/VM01_1.vmdk.
CBTMotion: disk clone failed, aborting Storage VMotion.
[msg.cbtmotion.disk.clone.failed] Disk clone failed. Canceling Storage vMotion.
To solve this I created a snapshot on the VM and committed it. After this the storage VMotion worked successfully.