There is a risk when plugging USB devices or other writeable storage media into a conventional operating system that the OS will write to the device, meaning a clean dump of the original media cannot subsequently be made. To enable as clean as possible a dump to be preserved - even in the case of used devices it is good to avoid making any further changes. Note that the write-protect switches that some SD cards doesn't actually prevent writes - if it works, it just tells the reader to not write to the card - not all readers will comply with this.
This method uses a forensics-oriented Linux distribution which defaults to blocking all devices in read-only mode to ensure as secure as possible an environment (without using a hardware write blocker) for imaging of writeable storage media.
Important Notes
Tools required
Basically the process is the same as Method 2, but you *do not* enable the system-wide write access, instead you just connect your PC to the network and copy your dump to another device.
Tools required
Process