[root@localhost scsi_host]# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 13G 6.2G 6.1G 51% /
tmpfs 931M 224K 931M 1% /dev/shm
/dev/sda1 283M 33M 236M 13% /boot
/dev/vx/dsk/mouryadg/vol1
200M 3.2M 185M 2% /mountpoint
[root@localhost scsi_host]# umount /mountpoint
^C
[root@localhost scsi_host]#df -h
Filesystem Size Used Avail Use% Mounted on
/dev/sda2 13G 6.2G 6.1G 51% /
tmpfs 931M 224K 931M 1% /dev/shm
/dev/sda1 283M 33M 236M 13% /boot
/dev/vx/dsk/mouryadg/vol1
13G 6.2G 6.1G 51% /mountpoint
[root@localhost scsi_host]# umount /mountpoint
UX:vxfs umount.vxfs:ERROR : V-326299: cannot umount /mountpoint:Invalid argument
UX:vxfs umount.vxfs:ERROR : V-326299: cannot umount /mountpoint:Invalid argument

you can not unmount a partition that is in use

use a live gparted dvd
http://gparted.org/

and PLEASE!!! use the code tags!!!

can you please tell me how to stop using the partition

Check your current working directory. Or run lsof on the mount point. And see who's using the mount point. Then get them off of using the mount point and try umount again.

or use the gparted live cd