Take your own advice, and read the 3rd paragraph of that website:
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Ok, let's say you are the lucky owner of an IBM Deskstar 75GXP with a capacity of 45 gigabytes and you wish to know whether you can safely use it inside a SilentDrive? in order to reduce the noise. Firstly, the IBM specification sheet shows that the drive runs at 7,200 rpm - a good start since if it had been 10,000 rpm or higher, it would probably have run too hot. Also we can note that IBM specify a maximum ambiant operating temperature of 55°C in common with most other manufacturers.
Now let's get some more detail - we are told that the power dissipation at idle depends on the number of disks (platters) in the drive and will be either 6.7 watts or 8.1 watts. We have a 45 Gb model which we are told has 3 data disks (platters) and therefore the correct figure for our drive is 6.7 watts. This is less than the 6.8 watts SilentDrive? guideline figure and therefore the drive should be fine while enclosed.
Had we owned a 60 or 75 Gb model, the figure would have been 8.1 watts which would have meant the drive would have run much hotter, too hot to be enclosed. Obviously if you have not yet purchased a hard drive, it would be a good idea to check compatibility in this way in order to aid your buying decision before purchase.