Q. How does Azure Site Recovery support the replication of VHDX to Azure when Azure does not support VHDX?
A. Windows Server 2012 introduced VHDX which was a new version of VHD that provided higher performance, greater resiliency and a new 64 TB maximum size. In Azure, the VHDX format is not supported at the time of this writing however it is possible to protect VMs on premises to Azure that use VHDX (providing they are less than 1TB in size). This is because the VHDX is actually converted to a VHD during failover process to Azure and the VHD is converted back to a VHDX when failing back from Azure to on-premises. This is all totally invisible to you.