Error Auto Sensing Secondary HDD

I have an XPS420 with two 500 GB SATA hard drives not in raid config. I am running Windows 7. The first HDD is my main drive, the 2nd is empty and occasionally used to store pics or docs. I am getting an error when I power on that says Error auto sensing secondary HDD. So I figured the HDD failed. I disconnected it and eliminated it from the BIOS and the computer came up fine. The next day, I got the same error message again even though it was disconnected. I reconnected the drive and could not power up beyond the message. I kept turning the computer on and off and it eventually fired up. Ran continuously for a week and both drives worked fine. I replaced the BIOS battery and re-flashed the BIOS thinking it may be something there but it did no good. I powered off and again, couldn't restart. For months now, I have to leave the computer running, 24/7 for the fear of not being able to power up again. I noticed in the BIOS that under SATA 0, there is no drive description. But under SATA 1, it has the WD500 listed as it should. Not sure why it's not finding the description. I fail to understand why the system will run constantly but upon booting, I run into this error message. Also, if I run diagnostics, it completes with no issues. I did notice, however, that if I F5 to continue, I get a message that states grubb rescue> And, I get the 1,2 and 3 lights on on the display panel. Not sure what that means at all. It looks like a DOS screen but I can't do anything after it says grubb rescue>. Any ideas, suggestions would be greatly appreciated. Possible drive controller failure? BIOS issues? HDD issues? Software? I'm at the point where I'm going to have to dump this whole system because I need something reliable but if I back up the system and the try to restore it to another new system with all the data and what not, I'm sure something just won't work right.