Also are there any errors in the ESX server in the the /var/log/vmkwarning.log file of the or the /var/log/messages.log file - something like below?

WARNING: SCSI: CheckPathReady:2941: CheckUnitReady on vmhba2:3:13 returned I/O error 0x0/0x2 sk 0x2 asc 0x0 ascq 0x0