When using hard disk based portable storage devices (e.g. digimate/vosonic/zen), do note that the hard disk needs a an air cushion for the disk head to float above the disk surface in order to work. So for high altitude trips above 3000m (e.g. trekking in Nepal) it is better to use solid state storage devices like SD/CF/etc.
The following is quoted from IBM tech support regarding their microdrive:
"The Microdrive does need "AIR" to float the heads and typically above 10,000 ft the mass of the air is too low and the drive requires a pressurized environment similar to an aircraft or spacecraft. At high altitude the air bearings begin to loose support from the air molecules needed to provide the "air bearing" for the Negative Air Bearing Surface (NABS) design of the head. If this "air bearing" is removed or lowered (as is the case with low density air at high altitudes) the head damages the media and you could have loss of data. The drive is vented to maintain equal pressure inside and outside to provide the air and to maintain the same pressure. This eliminates the need for sealed and rigid covers that can tolerate pressure differences.
The OEM Functional specification defines the warranty range for operating altitude as 3,000 M or 9,000 ft (3ft/M). If the customer is mountain climbing with a GPS or digital camera above 9,000 ft the drive might have problems. (Mt Fuji ~ +13,000ft, Mt Raineer ~ +14,000 ft). Please note, this is the operating environment. Non operation at high altitudes, including vacuum, have no ill effects on the microdrive. Within passenger aircraft, the cabin is pressurized to 9-10,000 feet hence the drive would experience no difficulty operating in an aircraft cruising at 35-45,000 ft ! "