Are hard drive disks airtight

Generally speaking, any "Green" drive tends to work well in external enclosures. Unless the enclosure has decent ventilation and an active fan (not too loud), I'd go with a 5400 RPM drive rated for better power conservation. In the larger sizes it'll still be faster than a smaller higher RPM drive. A 5400 1.5TB drive will generally be faster than a <500GB 7200RPM drive.

Honestly, the only time I'd consider a higher RPM drive is if I were using a NAS storage device for iSCSI or heavy network traffic (More than say 5 active users). If you have special needs, like video recording then there are drives geared towards that, but it really depends on your enclosure.

I had a similar problem using 4 of this cases and 2 of this hard drives. Only one of the 4 identical cases accepted a WD30EZRX drive. When attached through the other 3 cases, the computer reported a 2.2 TB hard drive and a 0.8 TB hard drive of unknown partition table – instead of a single 3 TB drive with intact GPT partition table. I found out that the vendor of the cases only supports hard drives up to 2 TB capacity (using that type of external case) and so I think I had just luck having 1 working case.

Long story short: Be sure that your case vendor supports hard drives of your desired size.