As from ESX 3.0 the drivers required to access an external USB hard-drive are not loaded by /etc/modules.conf. This was probably done to stop a beign error happening on boot-up. Never fear you can still load up drivers manually appropriate to your system. Use:

lspci -v | grep HCI

To list what type of USB host controller interface you have. There are 3 types

Based on the results of lspci -v | grep HCI you then load up the appropriate module (driver) with

modprobe usb-ochi or modprobe usb-uhci or modprobe ehci