Several times I have come across the situation when the NFS datastore on the VMWare ESXi host becomes unavailable / inactive and greyed out in the host’s storage list. However, the NFS storage stays available on the network level. Usually, it can be solved by removing the NFS storage and then adding it back.
However, it happens that when trying to remove such inactive NFS storage from the ESXi host configuration, it returns an error:
The resource datastore_nfs02 is in use.Call “HostDatastoreSystem.RemoveDatastore” form object “datastoreSystem-28″ on vCenter Server “xxxx” failed.
CannotRemove datastore ‘datastore_name’ because Storage I/O control is enabled on it. Correct it and re-try the operation

Based upon the error, the NFS storage cannot be removed due to the Storage I/O control (SIOC) is enabled. If you disable it, another error appears:
Datastore is not accessible
Solution
To properly remove the NFS storage from the VMWare ESXi host, you must directly connect the ESXi host server via SSH (you can enable SSH from vSphere Client).
Display the list of the NFS storages in the system:
esxcli storage nfs list |

A similar command in ESXi 4.x looks like this:
esxcfg-nas -l |
To unmount the NFS storage:
esxcli storage nfs remove –v datastore_nfs02 |
In ESXi 4.x command is as follows:
esxcfg-nas -d datastore_nfs02 |
Note. If the name of the NFS storage contains spaces, it has to be enclosed in quotes.

If the NFS datastore isn’t removed from the vSphere Client, click the Refresh button in the ESXi storage section (Configuration -> Storage).
Note. This has to be done on every ESXi host, where you need to remove the inactive storage.