Datastore Space Utilization - 'Other' File Types

Hi, All.

We have a SATA FC LUN, presented to 3 x ESX 4.1 hosts and 1 x ESXi 4.1 host (all fully patched) from an IBM DS4800 SAN. We wanted to provision some new test VMs and noticed that the provisioned space didn't add up the total of the VMs (see attached images):

The 'Performance' view is the most revealing. It shows 266GB of 'other' data on the datastore; the question is, what is it?

Querying the datastore via the vSphere Client, ESX console and PowerCLI reveals nothing. The totals match with the total of the provisioned VMs.

Any ideas what this could be? There are no snapshots, or large data dumps (ISOs etc) on the datastore as far as I can see.

Thanks,

Carl