If umount reports that a CD, USB drive, or other removable filesystem is busy, first find out which process is still using the mount point. Killing processes blindly can cause data loss or interrupt unrelated work.
Find what is using the mount
fuser -vm /mount/point lsof +D /mount/point
Close shells, file managers, editors, or applications using the filesystem, then try a normal unmount:
sudo umount /mount/point
If it is still busy
fuser -k can terminate processes using a mount, but use it only after reviewing the process list. A lazy unmount with umount -l detaches the filesystem now and cleans up references later; it is useful in specific recovery situations but should not be the first choice.
Avoid force-unmounting writable filesystems unless you understand the consequences. For removable storage, wait for pending writes to finish before disconnecting the device.