One way to find out the device letter is this: Before inserting the SD card into yout PC SD card slot, type ls /dev/sd*, which will give you the names of all hard disk partitions installed on your system. Then insert the SD card into the reader and redo the same command. The disk you have added is the one you want to use.
Added:
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When you put the SD card into the SD card reader, the system will automatically mount it because at the factory a vfat file system was put onto it. Before we can copy the Raspberry Pi image we must first unmount this partition:
umount /dev/sdX1
Copy the OS image to the SD card or USB stick with:sudo dd bs=4M if=2017-07-05-raspbian-jessie.img of=/dev/sdX status=progress conv=fsync