Asus A6J laptop: No bootable device, insert boot disc?

Hi there, my laptop ASUS A6J shows an interesting problem.

After any fresh OS installation (i.e. Windows 8, Vista or Linux), it boots immediately without any problem.
But if the laptop remains powered off for a tens of minutes, then it does not boot and shows the error:

No bootable device. Please insert boot disc and press any key to continue...

It sounds strange because the BIOS always reads the hard disk correctly.
The SMART functionality also does not show any issue, as well as various HDD tools used for diagnosis
(i.e. Hiren's CD, Windows check).

I have also tried to replace the hard disk, but the problem persists.

Thus I thought to a battery CMOS problem (since it is older than 10y), I replaced the CR2010 but the issue
remains exactly the same: cold boot does not work, warm boot is fine (i.e. by booting from a CD/DVD).

Now the laptop is disassembled on my lab.

What do you suggest to check?

Thanks for your help.

Have you try to reprogram the BIOS?