I find the easiest method is to look at what is mounted. If the SD card is inserted, it should be automatically recognized:

$ mount -l

Another way to find the mount(s) on the SD card is looking using the dmseg command. Ubuntu's documentation recommends using dmesg to find out which device the SD card is. Insert the SD card and run this:

$ dmesg | tail -20

You'll get a response similar to this:

dmesg output after SD card insertion

Before overwriting, you need to detach the disks that are mounted. In this case, sdd1 and sdd2 are the mounted partitions for device sdd, as shown in the last line of the image above.

In Linux, your disk devices are usually identified by looking in the /dev/ folder for devices starting with sd (for example sda, sdb, sdc). A partition is a division of the physical volume so it can be treated as different devices, and they can be identified by having incremental numbers (for example sdd has partitions sdd1 and sdd2):

$ sudo umount /dev/sdd1
$ sudo umount /dev/sdd2

After downloading the latest ZIP file of the Raspbian image from the Raspberry Pi website, you can validate it using the sha1sum tool:

$ sudo sha1sum ~/Downloads/2016-05-27-raspbian-
    jessie.zip
64c7ed611929ea5178fbb69b5a5f29cc9cc7c157
   /media/sf_Downloads/2016-05-27-raspbian-jessie.zip

If your return value matches the SHA-1 value on the Raspberry Pi Downloads page, unzip the file, and run the dd command to write your image to the SD card. Make sure you are using the correct device!

The dd command is for entire disks, so we reference sdd instead of the partitions sdd1 or sdd2. We don't want to duplicate things to partitions, we want to duplicate the entire disk, and whatever partitions your source has will be applied to the destination, our SD card.$ sudo dd bs=1M if=/path/to/raspbian-jessie.img of=/dev/sdd