Kill the processes using the filesystem and then unmount it.
use lsof
The following command finds out the processes accessing the partition/filesystem. You need to run the following command as root. Here, we use the mount point /mnt/data as an example.
# lsof | grep '/mnt/data'
It will output lines like
bash 17622 user1 cwd DIR 253,2 4096 2 /mnt/data
If you are sure that it is safe to kill the process, you may kill them by kill or kill -9. After the processes are killed, the filesystem will be able to be unmounted.