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fan or no fan?
Yes. I like them, others had rather save the cost. Some make more noise than the Mac. Newer drives use much less energy and thus create much less heat. Good ones have temp sensors that will shut down the mechanics if they over heat. Flip a coin a few times and take your choice.
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the fine print it's compatible with OSX10.7
Mostly, out-of-date marketing speak. Drives must be formatted for whatever OS they 'talk' with; they just come formatted for Windows, mostly. Many marketing folks have finally realized that they can save tech support phone calls by simply not specifying any OS or just saying "Mac OS". OTOH, they do have problems with the wide array of "Windows" OSs now available.
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pricing for enclosures from $24 to $64. What's the main difference?
Just like cars, vacuum cleaners, and dinning experiences. Sometimes you get benefits from the 'high priced spread'. Many times you just get higher priced marketers.
The important things to look for (in my humble opinion) are the newest chip sets (this can take research), length of warranties, manufacturer reputations, and massively good reports! The last three are usually available at he larger seller sites.
Just to throw in another monkey wrench, I'll be considering a NAS system (probably with multiple drives). Not wirelessly connected but with Ethernet. That way, I can have one box for both my wife and myself very easily and with higher connection speeds. I worry more about what would happen if she lost access to her data; she has but one back up drive; partitioned, but still one mechanical device.
The drives are the same, as usual, (a drive is a drive, mostly) they simply connect differently. Of course, with my luck, by the time she needs a new laptop, Apple will have eliminated Ethernet... I think they already have from the Air?