Q. Why can't I create a volume inside a Hyper-V VM larger than 64 TB?
A. In Windows Server 2012 and above the VHDX virtual hard disk format supports files up to 64 TB in size allowing volumes inside the VM to be up to 64 TB. I honestly cannot think of a requirement for a volume bigger than 64 TB however the question comes up if its OK to merge separate VHDX files together however this is not advisable for a number of reasons.
- You will not be able to backup the volume since VSS has a 64 TB limit as documented at https://support.microsoft.com/en-us/kb/2967756
- Hyper-V Replica will not work as it has a requirement for one-to-one relationship between file system and VHD in addition to the fact Hyper-V Replica uses VSS which also won't work
Long story short try to avoid volumes bigger than 64 TB!