Hard drive failures

[[TC]»Elmo«]

Seagate and WD are the best quality brands, followed closely by Samsung. Hitachi are the worst by some margin.
I have 2 1TB and 1 3TB Seagate (one's branded Maxtor) drives and they've had no problems so far. I've had a failure on my 1.5TB WD (see below), but its replacement is still gong strong.

Most likely either you've been unlucky, or there's something wrong with your system.

There's something known as Infant Mortality in most manufactured products, especially electronics. Basically, the vast majority of failures happen either in the first few months - the Infant Mortality period - because of manufacturing defects (usually <1 year, hence the 1yr basic warranty) or after several years when things start failing due to age.
With some kinds of hard drive failures, you only find out when you first write to/read from that part of the disk - I had that recently where some sectors a couple of hundred MB from the end of a 1.5TB WD drive were bad. Took me over a year to write enough data to fill it up that far to find out.

Regarding the environment: you said that your case is well ventilated - contrary to popular belief, a study by Google (they use many millions of hard drives) showed that HDDs actually like it quite warm - between ~30-40 degrees IIRC. If your room gets quite cool/cold and you have air from outside the case blowing directly on the drives, that may well cause problems. Much worse if it's cold AND humid.
It might be an idea to get some software (like speedfan) to monitor your drive temps.