All mobile forensic tools should prevent writing to the device except when it's necessary to gain access to the device. For example, it has become common practice to place an agent/file on some devices to get access to as much data as possible (this started way back with Windows CE devices).

There are a lot of mobile forensic tools you can choose from. Most will give you a free trial.  

Hi,

I guess UFED works as write blockers too.