Searching the WEB and finding this helped us get our volume unmounted without a reboot of the ESX server. I thought I would share what we did, it may help someone else with similar problem.
After a hardware failure of our iSCSI SAN we were unable to unmount datastore from ESX host. However we were able to 'kill' the parent processes that were hanging onto the datastore and rescan Storage Adapters which unmounted and removed the datastore from the ESX host. Some of the processes were from machines that were attached to the storage at the time of the SAN failure but no longer existed on the server and 'kill'ing those processes was key to getting ESX to unmount the datastore.
~ # esxcli storage filesystem list
Mount Point Volume Name UUID Mounted Type Size Free
------------------------------------------------- ------------------------ ----------------------------------- ------- ------ ------------------- -------------------
/vmfs/volumes/4cf65020-cdd99510-1fd7-842b2b05c775 esxisrv_localdisk 4cf65020-cdd99510-1fd7-842b2b05c775 true VMFS-5 140660178944 139948195840
/vmfs/volumes/51646b41-f9944c94-40a8-001018846202 Secondary_Raid_10_7k_vd5 51646b41-f9944c94-40a8-001018846202 true VMFS-3 1395595935744 860713123840
/vmfs/volumes/4f7f2302-d03c7288-67d4-00101884620a Raid_5_15k_vd1 4f7f2302-d03c7288-67d4-00101884620a true VMFS-3 7021786302599163750 1248136196516808178
/vmfs/volumes/4d7fc952-1264e9f8-07ff-001018846070 Utility Storage 4d7fc952-1264e9f8-07ff-001018846070 true VMFS-5 999385202688 676720541696
~ # esxcli storage filesystem unmount -p /vmfs/volumes/4f7f2302-d03c7288-67d4-00101884620a
Volume '/vmfs/volumes/4f7f2302-d03c7288-67d4-00101884620a' cannot be unmounted. Reason: Busy
~ # esxcfg-scsidevs –m
naa.6842b2b01c2d4300146d3b1e08686e34:3 /vmfs/devices/disks/naa.6842b2b01c2d4300146d3b1e08686e34:3 4cf65020-cdd99510-1fd7-842b2b05c775 0 esxisrv_localdisk
naa.690b11c000377c4e000001ff51649fa5:1 /vmfs/devices/disks/naa.690b11c000377c4e000001ff51649fa5:1 51646b41-f9944c94-40a8-001018846202 0 Secondary_Raid_10_7k_vd5
naa.6842b2b00066caec000084bb4f7e9e4e:1 :1 4f7f2302-d03c7288-67d4-00101884620a 0 Raid_5_15k_vd1
naa.6842b2b00066caec00000da24d7f5fac:1 /vmfs/devices/disks/naa.6842b2b00066caec00000da24d7f5fac:1 4d7fc952-1264e9f8-07ff-001018846070 0 Utility Storage
~ # vsish -e ls /storage/scsifw/devices/naa.6842b2b00066caec000084bb4f7e9e4e/worlds/ |sed 's:/::' |while read i;do ps |grep $i;done
8216 idle0
8881 helper41-4
8291 OCFlush
8878 helper41-1
8891 helper41-14
11326 11326 vmx /bin/vmx
11333 11326 vmx-vthread-7:termsrv09 /bin/vmx
12256 11326 vmx-svga:termsrv09 /bin/vmx
12258 11326 vmx-vcpu-1:termsrv09 /bin/vmx
12259 11326 vmx-vcpu-2:termsrv09 /bin/vmx
12260 11326 vmx-vcpu-3:termsrv09 /bin/vmx
8293 BCFlush
11333 11326 vmx-vthread-7:termsrv09 /bin/vmx
11327 vmm0:termsrv09
11329 vmast.11327
12257 11326 vmx-vcpu-0:termsrv09 /bin/vmx
12258 11326 vmx-vcpu-1:termsrv09 /bin/vmx
8267 helper19-0
12259 11326 vmx-vcpu-2:termsrv09 /bin/vmx
8883 helper41-6
8882 helper41-5
8879 helper41-2
8890 helper41-13
3018890 3018867 hostd-worker hostd
8887 helper41-10
8886 helper41-9
8877 helper41-0
1938877 1946811 vmx-vcpu-0:srv-app /bin/vmx
8892 helper41-15
3018892 3018892 nssquery /usr/libexec/hostd/nssquery
8880 helper41-3
8884 helper41-7
8889 helper41-12
3018889 3018867 hostd-worker hostd
8244 helper13-2
3018244 3018240 sfcb-vmware_aux /sbin/sfcbd
8888 helper41-11
3018888 3018867 hostd-poll hostd
8242 helper13-0
3018242 3018240 sfcb-vmware_aux /sbin/sfcbd
12260 11326 vmx-vcpu-3:termsrv09 /bin/vmx
8228 helper3-1
82287 82064 vmx-vthread-6:kvchecho /bin/vmx
82288 82064 vmx-mks:srvecho /bin/vmx
82289 82064 vmx-svga:srvecho /bin/vmx
8227 helper3-0
8229 helper3-2
82290 82064 vmx-vcpu-0:srvecho /bin/vmx
82291 82064 vmx-vcpu-1:srvhecho /bin/vmx
3018229 3018229 openwsmand /sbin/openwsmand
3018230 3018229 openwsmand /sbin/openwsmand
3018231 3018229 openwsmand /sbin/openwsmand
8863 helper39-0
8243 helper13-1
12255 11326 vmx-mks:termsrv09 /bin/vmx
~ # kill 11326
~ # kill 11327
~ # vsish -e ls /storage/scsifw/devices/naa.6842b2b00066caec000084bb4f7e9e4e/worlds/ |sed 's:/::' |while read i;do ps |grep $i;done
8216 idle0
8881 helper41-4
8291 OCFlush
8878 helper41-1
8885 helper41-8
8891 helper41-14
8293 BCFlush
8267 helper19-0
8883 helper41-6
8882 helper41-5
8879 helper41-2
8890 helper41-13
8887 helper41-10
8886 helper41-9
8877 helper41-0
1938877 1946811 vmx-vcpu-0:srv-app /bin/vmx
8892 helper41-15
8880 helper41-3
8884 helper41-7
8889 helper41-12
8244 helper13-2
8888 helper41-11
8242 helper13-0
8228 helper3-1
8227 helper3-0
8229 helper3-2
8863 helper39-0
8243 helper13-1
~ # esxcli storage core adapter rescan –all
~ # esxcli storage filesystem list
Mount Point Volume Name UUID Mounted Type Size Free
------------------------------------------------- ------------------------ ----------------------------------- ------- ------ ------------------- -------------------
/vmfs/volumes/4cf65020-cdd99510-1fd7-842b2b05c775 esxisrv_localdisk 4cf65020-cdd99510-1fd7-842b2b05c775 true VMFS-5 140660178944 139948195840
/vmfs/volumes/51646b41-f9944c94-40a8-001018846202 Secondary_Raid_10_7k_vd5 51646b41-f9944c94-40a8-001018846202 true VMFS-3 1395595935744 860713123840
/vmfs/volumes/4d7fc952-1264e9f8-07ff-001018846070 Utility Storage 4d7fc952-1264e9f8-07ff-001018846070 true VMFS-5 999385202688 676720541696