Thanks Combat Wombat,
I just have the 1 SATA drive that I mentioned, so I'm not sure that RAID would be the issue.
However, after an exhaustive search I found that remnants of a former OS (such as Linux) could be causing the issue (even after a quick format). So I am now wiping the drive with DBAN to see if this solves the problem. I will report the outcome.
P.S. If anybody out there wants to run DBAN make sure you have some time on your hands! Looks like mine is going to take about 7 or 8 hours, it's a 250 gig.