Dear Sir/Madam, my motherboard model is DH61WW. Its AA number is AAG23116-303. BIOS version is BEH6110H.86A.0109.2012.1221.1455. I cannot find the serial number as I brought the PC from a third-party retailer. Recently, I attached an external hard drive to one of the USB ports. Now, every time when I start up the PC, a BIOS message appears saying, "Invalid system disk. Replace the disk, and then press any key." But, when I just press 'ENTER' key on my keyboard, Windows get started, and the external hard drive is also working as usual. Could you please help me to get rid of aforesaid BIOS error message?