Today I am going to describe problem which happens very often on the virtual Windows machines, however I have seen people complying about this problem on the physical servers.
Problem: Windows is not recognizing correct space of the disk after extended it in the Disk Manager.
Than you noticed that yours disk needs more space, after extended space of the virtual disk in the Hyper-v you went to the disk manager and extend the disk. However Windows is still not recognizing space which you just added it.
Easy fix it for this.
Go to command prompt and enter the following commands:
C:\DISKPART>select volume # (select volume which has problem with recognizing space)