Why are file permissions retained when moving files within the same volume?
I have explained this in a blog post http://think-like-a-computer.com/2011/07/24/moving-files-on-the-same-ntfs-volume-does-inherit-permissions/ but it is also explained below.
When a file is copied, it has to create a brand new file and assign it a new set of permissions, so it gets the permissions from the parent folder as you know.
When a file is moved to another volume, what actually happens is that it is copied to the new volume and the old file is deleted. So the same process is repeated as above as it is a new file again and needs permissions set.
When the file is moved within the same volume, nothing really happens (at the disk level). It just changes the logical path location of the file. The actual data and physical file on the disk hasn't been touched or changed. Ever noticed when you move a 5GB file to another folder on the same drive, it is done almost instantly? This is why, because it actually hasn't moved but the pointer to where the file logically exists has changed. As it was not modified in any way, the permissions don't change also.
This is the reason for this behaviour.
Edit: Something I forgot to mention... The MS article isn't entirely accurate. MS quote:
By default, an object inherits permissions from its parent object, either at the time of creation or when it is copied or moved to its parent folder. The only exception to this rule occurs when you move an object to a different folder on the same volume. In this case, the original permissions are retained.
The above quote only applies to objects that have been given EXPLICITLY defined sec permissions (turn inheritance off). As mentioned in my comments, it is all about keeping the ACL entries as efficient as possible. Consider the following example:
To keep the explanation simple, let's say you have a folder set to allow users modify rights only. Below this, there're thousands of files and none of them have explicit permissions set. It isn't very efficient to create ACLs for each file as they are exactly the same perms so it sets ONE ACL entry for the folder. This next bit is very IMPORTANT to understand; the files themselves have NO ACL PERMS. So when you move any of these file into a new folder in the same volume, MS claims the perms move with it (as above quote). Ask yourself this....how? There were no perms on the file in the first place to move across. This is actually incorrect and I just tested it now to confirm it. Let's say the destination folder you are moving the file to has perms to allow the everyone group modify rights only. Well since the file has no ACL directly, it inherits the ACL of the parent folder. This means the perms have changed from users modify (old folder) to everyone modify (new folder).
Notice the difference?? This time around, moving a file to another folder in the same volume actually has changed the perms, something MS says it doesn't do. Have I just found a mistake in MS documentation since 2000 lol??
Now look at the same scenario when using explicit permissions. If you set explicit permissions on a file within this folder (inheritance turned off) which, for example, denies users read access, it now creates A NEW ACL entry specifically for this file. Now when you move the file to a new location, it has an ACL entry directly related to it. In this case, moving a file to a new location in the same volume RETAINS its permissions (as MS claims)!
When you are moving files within the same volume you are traditionally rearranging your file system. Altering the file permissions at the level of the directory could lock you out of that file the moment the move operation is finished. This is undesirable if, for instance, you just accidentally moved a file to a system, or a folder with special ownership permissions or otherwise protected. There would be no way of correcting the mistake other than taking ownership of the file (if you have the privileges), or logging with a privileged account. Considering normal day-to-day operation of a computer, you could find you had no control over your filesystem.
This behavior is common among most (if not all) operating systems making use of ACL. It guarantees normal filesystem operations within a volume by users and applications alike.
Conversely, when moving files between volumes you are traditionally giving a file away for control by something or someone else. It makes sense, as you well realize, for the file to then incorporate the target folder permissions, which will give the target the necessary permissions to then rearrange its own filesystem as they see fit.
Naturally this isn't always desirable. For which reason move and copy operations can be defined with special permission inheritance rules. From the same article:
To preserve permissions when files and folders are copied or moved, use the Xcopy.exe utility with the /O or the /X switch. The object's original permissions will be added to inheritable permissions in the new location.
To add an object's original permissions to inheritable permissions when you copy or move an object, use the Xcopy.exe utility with the –O and –X switches.