How to find offending process when getting error umount.nfs: /data1: device is busy

As root, use lsof ("list open files"):

[root@myserver /home/oracle]# lsof | grep '/data1'
extract    14041        oracle   25r      REG               0,23      2896 1235955379 /data1/goldengate/dirdat/et000000127 (storage1:/Oracle/myserver/data1)

You will see a list of processes. Terminate these with the kill-command:

kill 13859 14041 

After this is done, you can unmount the nfs file system:

 umount -f /data1