As [@LawrenceC][1] suggested, If your shell's current working directory is on the mountpoint path, you will get the "device is busy" error.

```
ubuntu@myip:~/efs$ sudo umount ~/efs
umount.nfs4: /home/ubuntu/efs: device is busy
```

`cd` to a location other than the mountpoint to resolve the error. 


 [1]: https://unix.stackexchange.com/questions/15024/umount-device-is-busy-why#comment20179_15024