How to umount a USB drive?
Suppose your usb drive is mounted to /media/usb
then it would be sufficient to do
sudo umount /media/usb
Suppose the your usb is /dev/sdb1
then you could also do
sudo umount /dev/sdb1
You may also have a look at the anwers of one of my questions, how to umount all attached usb devices with a single command: Umount all attached usb disks with a single command
You can also use
udisks --unmount /dev/sdb1
which does not require root.
For macOS:
diskutil umount /dev/sdb1
Better than using sudo
to unmount as root, just install the pmount package and do:
pumount /media/usb
As man pumount
says:
NAME
pumount - umount arbitrary hotpluggable devices as normal user
SYNOPSIS
pumount [ options ] device
DESCRIPTION
pumount is a wrapper around the standard umount program which permits
normal users to umount removable devices without a matching /etc/fstab
entry.