I was having some trouble with a couple iSCSI volumes in a two member ESXi cluster. Two of the volumes were display in the client adapater details as mounted, but the volumes were not showing as mounted in the Storage pane or with a df command in the CLI.

Logs to check in for a possible hint:

# grep -i volumename vobd.log
# grep -i volumename vmkernel.log

When I listed the filesystems from the CLI using the following, the volumes were not listed or obviously mounted:

# esxcli storage filesystem list

List iSCSI adapters configured:

# esxcli iscsi adapter list

A rescan of all the adapters did not work either:
Rescan adapter:

# esxcli storage core adapter rescan -a

To list all the devices and their world IDs:

# esxcli storage core device world list

To only list the world IDs tied to one device:
# esxcli storage core device world list -d mydeviceid

List all guests and their World ID:

# esxcli vm process list

Restart management services:

# services.sh restart

The vmkernel.log had a clue. A message stating that “Device mydeviceid detected to be a snapshot:”

# grep -i mydeviceid vmkernel.log

Listing the snapshots revealed the issue:
List snapshot:

# esxcli storage vmfs snapshot list

Both of my troubled volumes were listed as snapshots. I was told by VMware that this can happen if something changes in the META data of the iSCSI SAN/NAS. This was possible in my case, because I just updated my FreeNAS to the latest version. All that needed to be done was remove the snapshots. Once removed the volumes were mounted immediately.
Remove snapshot:

# esxcli storage vmfs snapshot mount -u “59b153b3-86f464ec-999d-a0d3c1f0cdf0”
# esxcli storage vmfs snapshot mount -u “59b1a680-bc18c507-831a-2c768a56eb24”