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This is a follow-up to this question.

I have two NTFS drives named disk 1 and disk 2. They are the same model (WD Red 4 Tb) and previously, they used to be clones of one another. To clarify, I used a docking station/cloner device to make a clone of disk 1 on disk 2. This means that they were basically the exact same device.

Following a change of backup strategy, I formatted and wiped disk 2 to a clean slate. I gave it a new letter and a new name. However, when I connect disk 1 and disk 2 simultaneously, Windows 10 only recognizes one of the drives. This is problematic because I would like to use my computer to copy files between the drives.

At this point, I have disk 1 full of data and disk 2 empty.

I tried the following (all after the formatting):

  • Connect disk 2 directly in SATA on the motherboard and disk 1 with the USB docking station. Windows only detects disk 2 and displays it as empty.

  • Change the letter of disk 2: set it to F, for example, while disk 1 is G. Result: when I connect disk 1 (even without disk 2 connected), it takes the letter F. As before, only disk 2 shows up.

  • Wipe the registry key to forget the letter and mounting point, then reconnect disk 2, disconnect disk 2, connect disk 1: disk 1 takes the same letter as disk 2. When I later connect the two simultaneously, only disk 2 is detected.

  • Using Ubuntu bash (WSL), check /mnt/. I see only one drive mounted, with only one letter.

What can I do to make disk 2 a different disk than disk 1 again? I would really like the two drives to show up simultaneously on my computer. I am also quite nervous to manipulate disk 1 while it is full of data.

C. Crt
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1 Answers1

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TL;DR

To online the offline drive, open Disk Management and set the offline drive to online. This will replace a relevant duplicate MBR disk signature or any relevant duplicate UUIDs to unique values. Note: Using the diskpart command's unique disk id= internal command is not necessary before setting the offline drive to online.

A more exhaustive explanation is given below.

Note that each NTFS volume contains a 8 byte volume serial number. These serial numbers do not have to be unique in order to get volume to mount under Windows. Although, it would probably be good practice to change the serial number, when one knows there are duplicates. Also, this may be required with mounting NTFS volumes under other operating systems.

Drives (often referred to as Disks), which use a MBR partitioning scheme, have a 4 byte MBR disk signature which is needs to be unique with respect to other drives, which are also using a MBR partitioning scheme. If two drives, with a MBR partitioning scheme, have the same MBR disk signature, then one drive may remain offline.

Table of Identifier Uniqueness Requirements for the MBR Partitioning Scheme

Identifier Must be unique? Can be changed by
diskpart command's
unique command?		 Automatically changed if
necessary when drive is
manually set to online?
MBR Disk Signature Yes Yes Yes
NTFS Volume Serial Number No No No

Drives (often referred to as Disks), which use a GPT scheme, have a drive UUID which is needs to be globally unique. Also, each partition has UUID which is needs to be globally unique. If two drives, with a GPT scheme, have the same drive or partition UUID, then one drive may remain offline.

Table of Identifier Uniqueness Requirements for the GPT scheme

Identifier Must be unique? Can be changed by
diskpart command's
unique command?		 Automatically changed if
necessary when drive is
manually set to online?
MBR Disk Signature No No No
Disk GUID Yes Yes Yes
Unique partition GUID Yes No Yes
NTFS Volume Serial Number No No No

Location of Identifiers

Table of Locations

Identifier Length
in Bytes		 Type of
Storage		 Location Offset
in Bytes
from Location		 Backup Offset
in Bytes
from Location
MBR Disk Signature 4 LSB Drive 440 none
Disk GUID 16 UUID Drive s+56 D-s+56
Unique partition GUID 16 UUID Drive 2*s+128*i-112 D-s-16496+128*i
NTFS Volume Serial Number 8 LSB Partition 72 N+72

Note the following:

  • LSB is an abbreviation for Least Significant Byte first. This means the bytes are stored in reverse order. See little-endian as explained in the Wikipedia article "Endianness". For example, an NTFS Volume Serial Number of 0x0001020304050607 would be stored as the hexadecimal byte sequence 07 06 05 04 03 02 01 00.
  • UUID is abbreviation for Universally Unique Identifier. This means the bytes are stored in mixed order. See the "Endianess" section of the Wikipedia article "Universally unique identifier". For example, an UUID of {00010203-0405-0607-0809-0A0B0C0D0E0F} would be stored as the hexadecimal byte sequence 03 02 01 00 05 04 07 06 08 09 0A 0B 0C 0D 0E 0F.
  • When the Volume Serial Number of a NTFS volume appears as a 4 byte value, what is shown is the last 4 bytes of the actual 8 byte NTFS Volume Serial Number.

Table of Variables

Variable Size Description
i The index of a GPT entry. The first entry has an index value of 1.
s The sector size of the drive in bytes.
N The size of the NTFS volume in bytes. The value for N can be computed by multiplying the size in sectors of the NTFS volume by s. This size in sectors is stored at byte offset 40 of the partition containing the NTFS volume. This 8 byte long size is stored in reverse order (LSB).
D Size of the drive in bytes.

References

David Anderson
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