studio 1535 boot for USB

Hi,

I am having a problem booting from a USB on studio 1535. The BIOS does not seem to check if a USB stick is plugged in on boot (I can tell, because the stick does not light up).

However, it works after the battery is depleted: for some reason, in these cases, POST lasts a very long time and checks for USB. I don't understand why it is so slow after battery depletion. I couldn't reproduce this during a regular boot after tweaking all relevant BIOS options.

Can someone tell me what I am doing wrong?

Thanks in advance