You get this error because the exfat filesystem is not installed in Ubuntu by default. exFAT is proprietary and patented by Microsoft.

Ubuntu 13.10 or higher

Since Ubuntu 13.10, this package is in the main repository. Just install exfat-fuse and exfat-utils:

sudo apt-get install exfat-fuse exfat-utils

Ubuntu 12.04

For ubuntu 13.04 and lower, you'll need a ppa to install the exfat support. Installation procedure:

sudo apt-add-repository ppa:relan/exfat
sudo apt-get update
sudo apt-get install fuse-exfat

If you see the error gpg: "tag:launchpad.net:2008:redacted" not a key ID: skipping during the apt-add-repository step, then you'll need to manually install the signing key and run the apt-get update and apt-get install steps after that:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 4DF9B28CA252A784

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Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:

Install Git and get the source code for Exfat-nofuse:

sudo apt-get install git
git clone https://github.com/rxrz/exfat-nofuse.git

Install Exfat-nofuse:

cd exfat-nofuse
make
sudo make install

Load the Exfat-nofuse module to the kernel:

sudo modprobe exfat_fs

^{Source:LinuxG}