When backing up a Hyper-V Virtual Machine running on Windows Hyper-V Server 2016 (Windows 10) or higher AND this Virtual Machine is running on outdated VM configuration version 5.0, then Changed Block Tracking information for this virtual machine cannot be retrieved and entire virtual disks are processed during backup.
Example of backup log where the problem is observed – both virtual disks are read from start to end instead of reading only changed sectors reported by CBT:
2018-03-29T18:13:32:428+03:00 8156 I00BD0000: Backing up virtual machine 'vm:7FF720C6-357C-4350-A558-C1EAB3E065E3?type%3dmshyperv'.
2018-03-29T18:13:42:733+03:00 8156 I00950000: Selected Changed Block Tracking mode: 'Enable and use CBT'.
2018-03-29T18:13:44:209+03:00 8156 I00950000: Changed Block Tracking has been enabled for virtual machine '(Gen2) Win2012R2'.
2018-03-29T18:20:47:516+03:00 8156 I00000000: I/O statistics for device '(Gen2) Win2012R2(IFACE_SCSI0:0)': 9383589376 bytes read, 0 bytes written.
2018-03-29T18:20:47:522+03:00 8156 I00000000: I/O statistics for device '(Gen2) Win2012R2(IFACE_SCSI0:2)': 52510720 bytes read, 0 bytes written.
2018-03-29T18:20:47:543+03:00 8156 I0135003B: Command has completed successfully. Command=Backing up virtual machine disks;
This situation may occur if you had Hyper-V 2012 upgraded to 2016, while the existing Virtual Machines configuration was not upgraded. Please see additional details in Upgrade virtual machine version article
Upgrade your Virtual Machines configuration to higher version as described in Upgrade virtual machine version article.