There is no problem of My documents folder on "C" drive except the following occasions. First one is due to size of the "C" Drive. Already it has windows installation files and lot of applications. If you store files frequently, there will be low space on that main drive. It may affect your windows to work properly.
Second one is security reasons. For instance your system has affected by viruses and malwares or your Windows OS have corrupted. In these situations, you need to reinstall or repair your windows OS. If you don’t have backup of My documents files already, that can’t be recovered and your data will be erased.
Now you may think change the destination of My documents drive to another one is safe and good thing. It can be done with following easy steps.
1. Right click the My documents icon and select Properties (You can see My documents folder on Desktop or My computer)
2. In the Target tab, you will see the current Location of the My documents folder. Click on "Move" Button and select your new destination like "D" drive or "E" drive or any of your system drives.
3. Finally Click on Apply button to move your files to new destination. You can revert this process by clicking Restore button. That’s all.
Note: You have some free space on the new disk, more than old My documents folder’s size.