Section 4: Volumes, Mount Points, and Drive Letters

The Big Picture: Many Names for One Thing

Every volume on your PC has more than one name. It is like a person who is "Harshil" to friends, "Mr. Kachhadiya" at the bank, and a long ID number in a government database. Windows gives each volume three kinds of names:

This section covers all three.

4.1 Drive Letter Assignment

What it is: A drive letter is a single letter, A to Z, followed by a colon, that points to a volume. Typing D:\ in Explorer opens whatever volume owns D:.

History:

• A: and B: were reserved for floppy disks in the first PCs. Even today Windows normally skips them for hard drives.
• C: was the first hard disk partition, because A and B were taken. The habit stuck. Windows is usually installed on C:, which is why C: is also the boot volume (Section 2) on most PCs.
• D: to Z: are handed out in order to other volumes: second partition, DVD drive, USB stick, and so on.

That gives you at most 26 letters (24 in practice once A and B are avoided), which is why mount points (4.2) exist.

Who assigns the letters? A part of the kernel called the Mount Manager (mountmgr.sys). When a new volume appears, it picks the next free letter. The important trick is that it remembers: it keeps a list of "this volume gets this letter" so your USB stick comes back as E: each time, if E: is still free. This list lives in the registry (the Windows settings database) under HKLM\SYSTEM\MountedDevices. (We only mention it so you know where to look. Do not edit it.)

New volume appears
      │
      ▼
Mount Manager looks in its memory list
      │
      ├─ known volume ──► gives back its old letter (if free)
      │
      └─ new volume ───► gives next free letter (D, E, F ...)

Changing a letter (read only here): In Disk Management you can right-click a volume and choose Change Drive Letter and Paths. Be careful: programs and shortcuts that point to the old letter will break. Never change the letter of C: (Windows will usually refuse).

Windows tip: Some volumes deliberately have no letter: the EFI partition, the recovery partition and the MSR (Section 3). Removing the letter from a data volume simply hides it from Explorer; the data remains.

4.2 Mount Points (Folder-Mounted Volumes)

What it is: A mount point attaches a volume to an empty folder on another NTFS volume instead of giving it a letter. Opening that folder shows the contents of the second volume.

Example. Your C: is small, and you add a big drive. Rather than getting E:, you mount it at C:\Games:

Without a mount point            With a mount point

  C:\                              C:\
  ├─ Users                         ├─ Users
  ├─ Windows                       ├─ Windows
  └─ Program Files                 ├─ Program Files
                                   └─ Games  ◄── this folder is really
  E:\  (a second drive)                         the big second drive
  └─ Games

When a program saves to C:\Games\Doom\save.dat, Windows silently sends the request to the second volume. The program never knows.

Rules to remember:

Why use it?

• You are out of drive letters.
• You want a tidy tree instead of many letters.
• You want to grow a folder without moving files: just mount a bigger volume there.

How to create one (reference only, not part of the lab): Disk Management → right-click the volume → Change Drive Letter and Paths → Add → Mount in the following empty NTFS folder.

How to list them safely: run mountvol with no arguments. It prints a help text and then the list of mount points on your PC with their volume GUID paths (Section 4.3). (Details of its output can differ between Windows versions.)

Warning: Backup and clean-up tools that walk a folder tree can follow a mount point into another disk. Deleting inside C:\Games deletes from the second drive. Know where your mount points are.

4.3 Volume GUIDs

What it is: Every volume also gets a GUID (a long unique number, introduced in Section 3). Windows can address the volume by that number with a volume GUID path:

\\?\Volume{a1b2c3d4-0000-1111-2222-333344445555}\

(That one is made up for the example.) The \\?\ at the start is a Windows prefix that means "treat this as a raw path, no clever adjusting."

Why it matters:

• It never changes when the drive letter changes. A letter can move from one volume to another; the GUID stays with the volume.
• It works for volumes with no letter, such as the recovery partition.
• Backup, BitLocker and system tools use it so they can find the right volume even if letters get shuffled.
• Mount points depend on it. When you mount a volume in a folder, Windows stores "this folder points to Volume{GUID}."

How the names connect:

       Volume{GUID}   ◄── the true identity, never changes
        ▲     ▲
        │     └── C:\Games   (folder mount point, optional)
        │
        └──────── E:         (drive letter, optional)

Where do you see GUID paths?

Tip: On old Windows versions you may also have seen \Device\HarddiskVolume3 in logs and Task Manager. That is the internal kernel name of a volume. It can change when disks are added, which is another reason GUID paths exist.

Summary