You say your HDDs won't POST, does that mean the computer hangs during POST, or do you get to the "Invalid System Disk Error" message after the BIOS has fully POSTed and tries to boot your system?
Have you tried booting from the CD/DVD drive (i.e. use a Windows or Linux boot CD)?
Have you moved your internal drive to a different SATA port? Have you tried using a different SATA cable and power cable (those power cables tend to have fragile connectors on them)?
Does your motherboard have a "Legacy Parallel ATA" setting, and does that solve anything?
Have you tried the HDDs in another system? Have you tried installing a different HDD in your system?
When you say HDDs plural, are you also talking about your external drive too? Is it eSATA or USB (I'm guessing eSATA)?
If both drives are SATA/eSATA and neither are showing up, I'm guess your SATA/RAID controller or chipset is bad. If you can see the drives on another computer, then its probably your computer. If you install a different HDD in your machine and it doesn't see it, then its definitely your computer.
If you've done all the moving and swapping and nothing works, might be time for a new motherboard. Of course, maybe you could use an add-on SATA/RAID card, but I wouldn't continue using a damaged motherboard, something else is bound to fail for whatever reason your SATA/chipset failed.