Mount A USB Memory Stick / Drive

List available devices

ls -l /dev/disk/by-uuid/

SDA1 will be your USB memory stick (unless you have multiple).  What it means:

The SD means SCSI device which also incoporates USB drives.
The A means the 1st drive found (the next would be SDB, SDC, etc)
The 1 means the first partition of the device (multiple partitions will give SDA2, SDA3, etc)

Create a directory to assign it to and apply ownership of it

Note that "usb" can be anything you want,

sudo mkdir /media/usb sudo chown -R pi:pi /media/usb

Mount the USB Drive

sudo mount /dev/sda1 /media/usb -o uid=pi,gid=pi

You can now access your drive using the path /media/usb/

Un-mount the USB drive

sudo umount /media/usb

E.g. if you want to remove it before the RPi is powered down