By default Windows sends an administrative alert when the amount of free space remaining on a hard disk drive falls below 10 percent. This percentage can be configured using this setting.
Open your registry and find the key below.
Create a new DWORD value called "DiskSpaceThreshold" and set it to the percentage of free disk space remaining before an alert is sent.
Restart Windows for the change to take effect.
System Key: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\LanmanServer\
Value Name: DiskSpaceThreshold
Data Type: REG_DWORD (DWORD Value)
Value Data: 0 - 99 percent (Default is 10)