External hdd changing device node mid-use

[Panayiotis Karabassis <panayk@gmail.com>]

I have an external usb 2.0 LG hard disk. It is part of an md array which 
is in turn part of an lvm array. The strange thing is that while the 
drive is in use, it will suddenly change device nodes, for example from 
/dev/sda to /dev/sdb. This will cause the md driver to mark it as 
faulty. This is very disruptive. Why is this happening and how to solve it?