Mounted readonly vhdx problems

If i mount vhdx as readonly and without  assigning drive letters WinApi and 
IOCLS calls in many cases, but not in all fails with NOT_READY Win32 Error (for example 
GetVolumeInformation ) and Incorrect Function[0x00000000] (for exmaple for 
IoctlDiskGetDriveGeometryEx)... if i mount it not readonly and wit drive letter 
- all works like with real disc drive, but that is not appropriate. Why does 
that calls fails for readonly vhdx  without  drive letter? Whar are the restrictions?