Sorry it seems I've confused the issue by mentioning HD led.
I realise that you were refering to the led on the 1640.
As I said the frequency of the led on the 1640 is not that important, it can vary due to s/w - hd activity etc.
Time & quality are the main considerations.
The HD led was mentioned simply as confirmation that activity can take place due to background services (anti -virus etc). Even though no other process is taking place. This is why some folks turn off all non essential services when burning.