[Win 7] External USB drive "Offline" or Not Detected when two similar USB devices are connected

How to Fix the Disk Signature Collision Problem in Windows 7 ============================================================

Disconnect all external drives EXCEPT the new drive.

Windows 7 comes with a command line utility called diskpart that can let you view and change the disk signature.

Open a command prompt as administrator. To do this in Windows 7, click the Windows start menu (the round Windows icon on the left bottom corner), type "cmd" (without the quotes), right click the "cmd.exe"
item that appears at the top of your menu, and click the line "Run as administrator". Do this even if you are already logged in as administrator, since on Windows 7, administrators run with reduced rights by default.

A black command prompt window will open. In Windows 7, the title bar of the window will tell you that you are running it as Administrator. If it does not, repeat the step above.

Type "diskpart" (without the quotes) into the window. (Note: for this and the other commands described here, you'll have to hit the ENTER key after you finish typing your commands for them to take
effect.)

Microsoft DiskPart will start. When it is ready, it will issue a "DISKPART>" prompt, allowing you to enter your commands.

Type "list disk" (without the quotes). This will list all the disks that are currently mounted (connected to the system). The disk will not have the usual names and labels that you're accustomed to from the Windows Explorer interface, so you will have to recognize them by their sizes.

Note that "list disk" actually lists the physical disks, and not the partitions that you may have assigned drive letters. This means that if you have 2 physical disks, with 3 partitions on each, so that you have drives C:, D:, E:, F:, G: and H:, "list disk" will only show "Disk 0" and "Disk 1".

To view the signature of a disk, you must first select it. To select a disk, type "select disk x" (without the quotes) where x is the number of the disk from your "list disk" display. When you type
(say) "select disk 1", DiskPart will respond by telling you "Disk 1 is now the selected disk".

Now type "uniqueid disk" (again, without the quotes). DiskPart will respond with the disk's signature, a series of hexadecimal digits.

To change the signature to some other number, type "uniqueid disk ID=[NEW SIGNATURE]" (without the quotes) where "[NEW SIGNATURE]"
stands for the new identifier you want for the disk (without the square brackets and without the quotes). To make things simpler you can try changing only one digit in the existing signature.
[ For more information on the command you can type "help uniqueid disk" ]

To quit DiskPart, type "exit".

Once you've quit DiskPart, type "exit" again to quit the Administrator Command Prompt.

Remove the external drive and re-plug it in. It should now detect without problems.

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