How to mount / access a USB flash pen drive on Raspbian Raspberry Pi

First, display the partitions already mounted by Raspbian itself:

pi@raspberrypi:~ $ df -h
  Filesystem Size Used Avail Use% Mounted on
  /dev/root 1.3G 980M 212M 83% /
  devtmpfs 214M 0 214M 0% /dev
  tmpfs 218M 0 218M 0% /dev/shm
  tmpfs 218M 4.4M 213M 3% /run
  tmpfs 5.0M 4.0K 5.0M 1% /run/lock
  tmpfs 218M 0 218M 0% /sys/fs/cgroup
  /dev/mmcblk0p1 60M 20M 41M 34% /boot

Then, display all the partitions that Raspbian has detected (but not necessarily mounted):

pi@raspberrypi:~ $ cat /proc/diskstats | grep -v ram | grep -v loop
  179 0 mmcblk0 1726 1437 88794 5200 747 480 10025 4830 0 5530 10010
  179 1 mmcblk0p1 84 1 1624 100 1 0 1 10 0 100 110
  179 2 mmcblk0p2 1582 1436 86250 4960 746 480 10024 4820 0 5420 9760
  8 0 sda 221 0 3496 350 0 0 0 0 0 260 350
  8 1 sda1 132 0 1920 230 0 0 0 0 0 160 230

You should see the partitions for the SD card listed as mmcblk0p? where ? in the partition number.

You should see the partitions for the USB flash pen drive listed as sda? where ? in the partition number. If you have more than one USB flash pen drive connected to your Raspberry Pi, you will see sdb for the second drive, sdc for the third drive etc.

Make a directory to use as our partition mount point:

pi@raspberrypi:~ $ mkdir /tmp/mnt

Mount the partition using this mount point:

pi@raspberrypi:~ $ sudo mount /dev/sda1 /tmp/mnt

Check that the mount has been successful:

pi@raspberrypi:~ $ ls -la /tmp/mnt/
  total 48
  drwxr-xr-x 6 root root 4096 Feb 5 20:58 .
  drwxrwxrwt 8 root root 4096 Feb 6 12:40 ..

The USB drive is now ready to be used.

NOTE: Before removing the USB drive, remember to unmount it, otherwise the drive may become corrupted:

pi@raspberrypi:~ $ sudo umount /tmp/mnt