unable to umount device is busy use lsof fuser umount -l

while we want to increase the partition size or remove the partition we try to umount the particular partition.In that time when we use umount  the particular partition it is unable to umount we will get the message.
Device is busy.

For an example
#umount /dev/sda1
if we get anything unable to umount busy
umount: /: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
fuser identify process which are using the file and socket.
# fuser -m /dev/sda1
it will show the list of process which are things using these disk
/dev/sda1: 2400
 1rce 2rc 3rc 4rc 5rc 6rc 7rc 8rc 9rc 10rc 11rc 12rc 13rc 14rc 15rc 16rc 17rc 18rc 19rc 20rc 21rc 22rc 23rc 25rc 26rc 27rc 28rc 29rc 30rc 36rc 37rc 39rc 41rc 44rc 45rc 46rc 47rc 48rc 49rc 233rc 234rc 235rc 236rc 309rc 310rc 351rc 380rce 384rce 524rce 526rc 545rce 587rc 594rc 601rc 702rc 837rce 848rce 859rce 860rce 866rce 870rce 871rce 881rce 883rce 953rce 978rce 1099rce 1234rce 1270re 1274re 1287re 1288re 1290re 1292rce 1294rce 1295rce 1343rce 1347rce 1368rce 1440e 1521re 1529rce 1548rce 1578rce 1581rce 1582rce 1587rce 1591rce 1596rce 1601rce 1606rce 1612rce 1614rce 1616rce 1617rce 1618rce 1619rce 1620rce 1622rce 1624rce 1625rce 1630rce 1636rce 1706rce 1723rce 1728rce 1729rce 1732rce 1734rce 1736rce 1752rce 1753rce 1754rce 1755rce 1756rce 1757rce 1768rce 1770rce 1773rce 1782rce 1794rce 1815rce 1848rce 1851rce 1852rce 1899rce 1946rce 1950rce 1954rce 2217rce 2274rce 2318rce 2374rce 2383rce 2400rce 2591rce 2592rce 2608rce 2617rce 2750rce 2773rce 2774rce 2845rce 2854rce 2862rce 2880rce 2901rce 2936rce 3250rc 3251rc

These device is using the above process.
#kill -9 pid
 Using kill command kill that process what are process occupy that device.
then try it.
other wise we can use lazy umount command | -l refers to the lazy
#umount -l /dev/sda5
if you still face the problem
check whether that particular partition is used by the nfs.
If it is used by the nfs then just shutdown the nfs service.
Then try it. It will work.
[root@master ~]# /etc/init.d/nfs stop
Shutting down NFS mountd: [ OK ]
Shutting down NFS daemon: [ OK ]
Shutting down NFS quotas: [ OK ]
Shutting down NFS services: [ OK ]
[root@master ~]# umount /dev/sda5
 lazy umount it will cleanup all reference of the file system and it is not busy any more.