When dealing with directory structures containing millions of files across multiple levels in an NTFS filesystem, traditional GUI methods like Windows Explorer become impractical. The recursive traversal behavior during ACL modifications creates significant performance bottlenecks.

The most efficient approach involves using command-line tools that can modify permissions without unnecessary file system traversal:

icacls "C:\TopLevelDir" /grant "DOMAIN\GroupName":(R) /T /C /Q

Key parameters:
/T - Recursively applies to all files and subfolders
/C - Continues despite errors
/Q - Quiet mode (no feedback)

For more granular control, PowerShell provides better options:

$acl = Get-Acl "C:\TopLevelDir"
$rule = New-Object System.Security.AccessControl.FileSystemAccessRule(
    "DOMAIN\GroupName",
    "Read",
    "ContainerInherit,ObjectInherit",
    "None",
    "Allow"
)
$acl.SetAccessRule($rule)
Set-Acl -Path "C:\TopLevelDir" -AclObject $acl -ErrorAction SilentlyContinue

After setting permissions, enable network sharing efficiently:

New-SmbShare -Name "ShareName" -Path "C:\TopLevelDir" 
    -ReadAccess "DOMAIN\GroupName" 
    -FolderEnumerationMode AccessBased

If inheritance is broken, reset it before applying new permissions:

icacls "C:\TopLevelDir" /reset /T /C /Q
icacls "C:\TopLevelDir" /grant:r "DOMAIN\GroupName":(R) /T /C /Q

• Run operations during off-peak hours
• Disable antivirus scanning during bulk operations
• Consider processing in batches for extremely large trees

When dealing with massive directory structures on NTFS (especially common in enterprise Windows Server environments), the standard GUI approach becomes painfully inefficient. Windows Explorer's security tab initiates a full recursive traversal of the entire directory tree - for structures containing millions of nodes, this can take hours or even days to complete.

The inefficiency stems from how Windows handles permission inheritance by default. Even when you're just adding a single permission entry at the root, the system wants to propagate these changes throughout the entire hierarchy. The previous admin might have also disabled inheritance at various levels, forcing Windows to check every single node.

For bulk operations, the icacls utility is vastly more efficient than the GUI. Here's the optimal command for read-only access:

icacls "D:\MassiveDirectory" /grant "DOMAIN\GroupName":(R) /T /C /Q /L

Parameter breakdown:

• /T - Recursive operation (still needed but faster than GUI)
• /C - Continue despite errors
• /Q - Quiet mode (no output)
• /L - Work on symbolic links themselves

If the directory has broken inheritance, reset it first:

icacls "D:\MassiveDirectory" /reset /T /C /Q
icacls "D:\MassiveDirectory" /grant:r "DOMAIN\GroupName":(R) /T /C /Q

For network access, you'll want to:

1. Set share permissions separately (via Computer Management)
2. Keep NTFS permissions as the primary control
3. Use this PowerShell command for share creation:

New-SmbShare -Name "DataShare" -Path "D:\MassiveDirectory" -FullAccess "DOMAIN\AdminGroup" -ReadAccess "DOMAIN\ReadOnlyGroup"

• Run operations during off-peak hours
• Disable antivirus scanning during the operation
• Consider breaking very large directories into logical subtrees
• For frequent operations, script the process with error handling

If users still can't access files:

# Check effective permissions
icacls "D:\MassiveDirectory\file.txt" /findsid "DOMAIN\UserName"
# Check share permissions
Get-SmbShareAccess -Name "DataShare"