umount — Unmount a Linux File System

umount detaches a mounted filesystem from the Linux directory tree. It is commonly used for USB drives, external disks, network mounts and temporary mount points.

Check mounted filesystems

findmnt
df -h

Unmount by mount point

sudo umount /mnt/backup

This unmounts the filesystem currently attached at /mnt/backup.

When the device is busy

lsof +f -- /mnt/backup
fuser -vm /mnt/backup

If unmounting fails with a busy message, check which process is using the mount point before closing files or stopping services.