How to unmount a busy device

YES!! There is a way to detach a busy device immediately – even if it is busy and cannot be unmounted forcefully. You may cleanup all later:
umount -l /PATH/OF/BUSY-DEVICE
umount -f /PATH/OF/BUSY-NFS (NETWORK-FILE-SYSTEM)

NOTE/CAUTION

These commands can disrupt a running process, cause data loss OR corrupt open files. Programs accessing target DEVICE/NFS files may throw errors OR could not work properly after force unmount.
Do not execute above umount commands when inside mounted path (Folder/Drive/Device) itself. First, you may use pwd command to validate your current directory path (which should not be the mounted path), then use cd command to get out of the mounted path – to unmount it later using above commands.

If possible, let us locate/identify the busy process, kill that process and then unmount the samba share/ drive to minimize damage:

lsof | grep ‘‘ (or whatever the mounted device is)

pkill target_process (kills busy proc. by name | kill PID | killall target_process)

umount /dev/sda1 (or whatever the mounted device is)