Quick Answer for Advanced Users
Run net use * /delete in an elevated Command Prompt, then remap the drive. This clears the cached size display.
Why This Happens
Windows caches the free space value from a network drive when you first map it. If the server's actual disk size changes—someone adds a file, deletes stuff, or the NAS gets a new volume—Windows doesn't refresh that cache. You end up staring at a free space number that's hours or days old. This bug is most common on Windows 10 (1909 and later) and Windows 11 when connecting to SMB shares on Synology, QNAP, or older Windows Server boxes. I've seen it myself on a QNAP TS-453D: the NAS showed 500 GB free, Windows said 10 GB.
Step-by-Step Fix
Step 1: Close File Explorer
Close all Explorer windows. Don't skip this. If a window is open, the drive stays locked.
Step 2: Open Command Prompt as Admin
Click Start, type cmd, right-click Command Prompt, choose "Run as administrator." Accept the UAC prompt. You'll see a black window with a flashing cursor.
Step 3: Disconnect the Problem Drive
Type net use Z: /delete (replace Z: with your drive letter). Hit Enter. You should see "Z: was deleted successfully." If it says "The network connection could not be found," you either typed the wrong letter or the drive isn't mapped. In that case, skip to Step 4.
Step 4: Remove All Mapped Drives (If Unsure)
Type net use * /delete and press Enter. It asks "Do you want to continue running this operation?" Type Y and hit Enter. This disconnects every mapped drive. After that, close the Command Prompt.
Step 5: Remap the Drive
Open File Explorer. Click "This PC" on the left. On the top toolbar, click "Map network drive." Choose a drive letter (same as before or new one). In the Folder field, type the full UNC path, like \server\share. Check "Reconnect at sign-in" if you want it persistent. Uncheck "Connect using different credentials" unless you need a different login. Click Finish. If it asks for credentials, type them.
Step 6: Verify the Free Space
Right-click the drive in File Explorer, choose Properties. The free space should now match the server's real value. If it still looks wrong, wait 30 seconds and hit F5 to refresh. Sometimes the cache takes a moment.
Alternative Fixes If the Main One Fails
Fix 1: Reboot the Network Adapter
Open Control Panel > Network and Sharing Center > Change adapter settings. Right-click your active network connection (Ethernet or Wi-Fi), choose Disable. Wait 10 seconds. Right-click again, Enable. Then remap the drive as in Step 5 above. This clears network-level caches.
Fix 2: Use IP Instead of Hostname
When mapping the drive, use the server's IP address instead of its name. For example, \192.168.1.100\share instead of \nas\share. This bypasses DNS cache issues that sometimes freeze the size display. I've had to do this on a Windows 11 machine connecting to an old Netgear ReadyNAS.
Fix 3: Disable Folder Redirection in Group Policy (For Domain Users)
If you're on a corporate network, your IT might have set folder redirection. Open gpedit.msc (Pro/Enterprise only) and go to User Configuration > Administrative Templates > Windows Components > File Explorer. Find "Remove the user's folder redirection UI." Set it to Enabled. This forces Windows to use the server's raw values instead of cached ones. Reboot after.
Fix 4: Change Registry Value (Advanced)
Back up your registry first. Open Regedit. Go to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\PrefetchParameters. Create a new DWORD (32-bit) named EnableSuperfetch. Set it to 0. Reboot. This disables Superfetch, which sometimes jams up network drive size reads on older systems. Don't do this on a laptop with a slow HDD—it'll slow down local file browsing.
Prevention Tip
Don't let Windows auto-map drives at startup. Go to File Explorer, right-click the drive, choose Disconnect. Then map it manually when you need it. That keeps the cache fresh. If you need it persistent, set up a startup script that runs net use Z: \\server\share /persistent:no and remaps it fresh each login. I do this on all my clients now—no more wrong-size calls.