it apeared only warm, but when I removed the drive it was actualy hot.
OK here in lies the possable cause of the failure of the HDD..
If the system is running 24/7 and the HDD is a 7200 RPM type the bearings will get extreemly hot and if allowed to run like this for an extended period of time the HDD will become noisey and fail.
Just taking the hdd out of the drivebay has also remove a inbuilt heatsink (heat transfer mechanism). this is also why it got hotter...

It is an excellent Idea to have a low volume fan circulating air around the HDD (even with 5400rpm models) when running for periods of greater than an hour.. this will keep the temps down...
useing a program like Everest (there was a demo version) will give you the hdd temps.. keep them below 40deg Celcius mine are typicly 25-28..