Did the USB box require any drivers to be installed prior to pluggin it into the PC?
Have you set the drive to "Master" or did you leave it on cable select?
Check the power connectors are all connected correctly.
Do you have any other USB devices plugged in? If so, try disconnecting them,
Also try plugging it into a different USB port.
My work has a whole bunch of DELL PC's (where's the sick symbol?) and they have 2 ports which can only be used for keyboards or mice, and nothing else works in those ports. (one pt at front, one at back).
Does the assigned USB drive letter for the NEC3540 conflict with any other devices (mapped network drives or HD drive partitions / etc)
It is standard for USB/Firewire enclosures to be reconised as a "USB/Firewire" device, but the NEC3540 will be listed seperately in the "CDROM" category.