Only 1 Compact Flash Card Can Be Use – Others Are Ignored

Why only one Compact Flash Card mounts—and how to fix disk signature collisions.

The most common reason why only one Compact Flash Card is usable when multiple cards are connected is a device signature collision.

If you are dealing with bootable devices and see this problem, the collision is almost certainly the cause. If the devices aren’t bootable, the details below may not apply.

What is a Compact Flash Card signature collision?

A disk signature is a unique identifier stored in the Master Boot Record (MBR) of any bootable device. It allows the operating system to distinguish between drives. When two devices share the same signature, Windows will block the second device from mounting. This can happen with Compact Flash, SD, microSD, or USB flash drives.

On Windows 7, 8, 8.1, and 10, the system disables the duplicate device and shows messages such as:

“The boot selection failed because a required device is inaccessible”
or
“This disk is offline because it has a signature collision.”

What causes the collision?

The most common cause is cloning. Using a Compact Flash Card duplicator copies not only the data but also the MBR signature. When both source and target are connected to the same computer, Windows detects the duplicate and blocks one drive.

How to fix a Compact Flash Card disk signature collision

There are two main methods: DiskPart or third-party partitioning software.

DiskPart Method

1. In the Windows search bar, type DiskPart and launch the utility.
2. At the prompt, type list disk to view connected drives.
3. Identify the problem drive by checking its capacity.
4. Select it using select disk # (replace # with the disk number).
5. Type uniqueid disk to display its current signature.
6. Assign a new unique value, for example: uniqueid disk ID=A53AEBE9.
7. Return to Disk Management and bring the drive online.

If the command fails, the MBR may not contain a changeable signature. In that case, reflashing the device with bootstrap code may be required. A step-by-step guide can be found here.

Partition Wizard Method

The easier option is to download the free Partition Wizard. This tool automatically detects disk signature collisions and updates the MBR with a unique identifier. It requires no guesswork and works instantly.

For further technical reading, you can explore the background of disk failures and collisions on Wikipedia.