Starting with SP5, up to 255 LUNs are supported, but they are not enabled.
To enable large LUN support, use Regedt32 to navigate to:
where <Driver_Service_Key> is your SCSI driver name and <\[Number\]> is the SCSI adapter number (0 - 1st, 1 - 2nd, etc..).
On the Edit menu, Add Value name LargeLuns, a type REG_DWORD, and set the data value to 1.
NOTE - If the LargeLuns is added to the Parameters subkey and NO <\[Number\]> subkeys exist, it applies to all <Driver_Service_Key> adapters.
NOTE - Deleting LargeLuns, or setting it to 0, disables large LUNs.