Whenever I access my external seagate HDD, my screen “wobbles” for the duration of the spin-up time. It’s really hard to explain it, it’s like horizontal parts of the screen distorting and moving about a bit for a few seconds when the HDD is accessed. All the colours are intact, it’s just that it moves about a bit.
I am using an LG 22" LCD screen and a 8800GT, it is connected through a RGB cable but uses a converter to plug into the GPU.
Let me try and explain it better. It is barely noticeable, it’s usually in the middle of the screen and it gives an “emboss” effect for about 2 seconds whilst the external HDD is spinning up. It’s really hard to explain.
The only time I have experienced something like this is when the external device was poorly shielded and too close to the Monitor.
But it shouldn’t happen to LCD screens because there is no electron beam and magnetism involved. Only thing I can think of is the HDD affecting the power supply when spinning up. Is your HDD drawing power from the USB port or does it have its own power supply?