Before you can use a USB hard drive in the Linux operating system (such as Ubuntu, Kubuntu, Fedora or Linux Mint) you must first "mount" the drive. This is a process where you associate the drive with a folder on the computer so that the operating system can recognize the drive and allow you to write files to and from the folder that it is associated with.
Log in to your operating system and open a terminal shell from the desktop "Terminal" shortcut.
Type "fdisk -l" to see a list of drives on your computer and to get the name of the USB hard drive (this name is usually "/dev/sdb1" or similar).
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Create a folder to associate with your USB hard drive by typing "mkdir /mnt/usbdrive" and pressing "Enter."
Type "mount -t /dev/sdb1 /mnt/usbdrive" and press "Enter" to mount your USB hard drive to the folder you created.