Like other modern versions of Linux, Debian's default GNOME desktop is configured to automatically mount USB storage devices as soon as they are plugged in, followed by a prompt to either view the files in the file manager or to eject the device. If the option to automatically mount USB devices has been disabled, or if the device has been unmounted, you can manually mount it by using either the terminal or the GNOME Disk Utility.
Mounting With the Terminal
Insert your USB device, then start a terminal session by clicking "Activities" on the GNOME panel, typing "terminal" and clicking the resulting icon.
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Type "su" followed by the "Enter" key to log in as the root, or administrator, account. Note that terminal commands are case-sensitive and the examples should be entered without the quotes. Type the account's password when prompted, then press the "Enter" key.
Type "fdisk -l" and press "Enter" to find the device name of your USB drive. If your drive is 8GB, for example, it will be listed by size as "8004 MB." A 500GB drive will be listed as "500 GB." On the same line, just before the size of the drive, you will find the device name. It will look similar to "/dev/sdb/."
Type the command "file -s /dev/sdb" and press "Enter" to find the file system of the drive, replacing "/dev/sdb" with the correct device name. The FAT32 file system will be listed after "OEM-ID" as "mkdosfs" and the NTFS file system will be listed as "NTFS." If "EXT4" is listed in the results of the command, the drive uses the Linux EXT4 file system.
Create a mount point for your device by typing "mkdir /mnt/external" and pressing "Enter." A mount point is simply a directory where the files from your drive will be linked so that you can locate them. The directory name, "external" in the example, can be any name that you would like, just avoid using spaces and remember that the name is case-sensitive.
Mount the device with the "mount -t vfat /dev/sdb /mnt/external" command, followed by the "Enter" key. Replace "sdb" and "external" with the appropriate device and directory. Replace "vfat" (used for FAT32 file systems) with "ntfs-3g" for NTFS file systems or "ext4" for EXT4 file systems. To unmount the drive when you are finished, type "umount /dev/sdb" and press "Enter."
Mounting With the Disk Utility
Start typing "Disk Utility" in the "Activities" menu and click the resulting icon.
Click on your USB drive, which will be located in the "Peripheral Devices" section of the left pane. Information about the device will be displayed in the right pane.
Select the "Mount" option from the "Volumes" section to mount the drive. When prompted, choose "Open in Files" to start the file manager. When you are finished using your drive, you can either click the eject icon in the left pane of the file manager or click "Unmount" in Disk Utility.
Debian typically offers KDE, XFCE and LXDE as alternatives to the default GNOME desktop. In KDE, after inserting the USB drive, start the Dolphin file manager and double-click on the device's name in the left panel to mount it and view the files. When a USB device is inserted in XFCE, an icon will automatically be placed on the desktop. Right-click the icon and choose “Mount” to mount the drive. In LXDE, after you insert a USB drive, the system will mount the drive and offer to open the file manager to view the files.
Be careful to enter all commands correctly when you are logged in under the root account in the terminal. An incorrect command entered with full administrator privileges could cause serious damage to the file system.