Windows crashing - IRQL Not less or Equal error - Where to start

This has been a nightmareof a problem.

First for the client I installed a new external hard drive. Same model as one they already have (freecom).

They told me Windows has started crashing since installation of the new USB hard drive (intially told me FAT32 error) so I went and upgraded external drives to NTFS (although internal C drive still FAT32).

Turned out this FAT 32 message was not an error but Windows checking the drive ater reboot.

Actual error message is a bluue screen which all I have is contains the message

IRQL Not Less or Equal

I've googled this and there is a lot of information. I have seen the error happen when I remotely logge dint (it crashed) although I can;'t see the actual error on screen of course.

It definelty at least occurs when doing disk operations (e.g. was importing a large library of music into Itunes on the external drive (note the ituens library is also on the same disk).

When I try this remotely it seems to crash half way throguh.

Where do I start wiht this error. I've tried so much.

Thanks