Disk's active time is 100% without writing

I have vDisk as VirtIO in Windows 10 VM. I have set its capacity to 2TB and its current allocation is 39GB. So, the problem I see is when I copy some data in amount about or above 1.5GB onto it, data starts writing onto it with a good speed of above 100MB/s; however, after a short while and before all data is written, the disk stops writing ANY data but its Active Time is at 100% at that time. After about 15-25 seconds, data continues to write. But the same keeps happening over-and-over again until all data is copied! What is wrong?
 

 <disk type='file' device='disk'>
      <driver name='qemu' type='raw' cache='writeback'/>
      <source file='/mnt/user/Media/Videos/someDiskName.img' index='2'/>
      <backingStore/>
      <target dev='hde' bus='virtio'/>
      <alias name='virtio-disk4'/>
      <address type='pci' domain='0x0000' bus='0x05' slot='0x00' function='0x0'/>
    </disk>