It is really strange that the both HDDs are detected by BIOS but not by diskpart. Also it is strange that the computer does not beep when booting.
You said "They also are using the same cables". Can you use different cables? Can you attach the drives to a different SATA port as well? I guess it will not hurt to attach the drives to a different computer. Though, I doubt that you have two faulty drives.
Is this the latest BIOS version for your MOBO? If it is not, you could consider updating your BIOS. You can reset the CMOS as well: http://www.howtogeek.com/131623/how-to-clear-your-computers-cmos-to-reset-bios-settings/
What is your MOBO? When one's system doesn't beep at start, more or less, there are two causes for that: 1st one - there is something wrong with the MOBO's speaker, 2nd one - there is something wrong with the MOBO.
D_Know_WD