The Sony CyberShot DSC-P51 is a camera that was first introduced in 2002. It was quite popular for its small size, ability to take 2.0 Megapixel photos, and its MPEG video recording capability. Although this camera has long since been out of production, if you are still trying to use it it is quite conceivable that you could have some issues with the camera's memory stick capabilities.

Wrong Format

The CyberShot DSC-P51 was designed to only accept Memory Sticks, which are no longer produced. This format has been succeeded by Memory Stick Pro and the Memory Stick Pro Duo. The Duo format is wholly incompatible with the CyberShot DSC-P51, but the Memory Stick Pro format can work with the CyberShot DSC-P51a if a special patch is downloaded so the camera can read the card.

Wrong Size

The CyberShot DSC-P51 is only able to process Memory Sticks that come in 4-128 MB sizes. If you are using a Memory Stick PRO with emulation software installed, the size needs to be less than 128 MB, otherwise the camera will not be able to read the card. Memory Stick PROs can reach sizes up to 4GB in size, so it is important to double check the size before using it in the camera.

Placed in the Wrong Way

Although it may seem very simple, sometimes the easiest answers are quite often the correct ones. If your memory card isn't working, take it out and make sure it is inserted with the logo facing out towards the memory card door. if it is not, then the memory card has been placed in backwards and will need to be flipped around before it will work correctly.

Source:

Sony DSC-P51 Review

Memory Stick Information