Scott Fraser wrote:
> I've been told that hard drives are hermetically sealed for dust
> protection, so I presume that air pressure changes won't be
> experienced within the drive mechanism.
The only sealed, pressurized hard drives I know of are for military
use in unmanned aircraft and missiles that lack pressurized airspace.
Hard drives available to the rest of us all have a small hole on the
bottom that is open to the surrounding air. The hole is backed by a
filter to block the entrance of dust, but the filter allows the
passage of air between the inside and outside of the drive. The hole
is often marked by a sticker that says, "Don't block this hole!" The
pressure inside the drive adjusts to match the pressure outside
through the hole.
During read and write operations, the heads of hard drives ride
microscopically close to the surface of the turning disc. Air
pressure helps create an air bearing that keeps the heads from
contacting the disc surface. That's why disc manufacturers include
warnings not to use the hard drive above 10,000 ft (about 3000 m).
Above this elevation, the air bearing effect is reduced to the point
where head crashes are more likely.
Lots of people use devices equipped with small hard drives, such as
some iPod models, at altitudes above 10,000 ft. But doing so comes
with an increased risk of disc failure. You have no one to blame but
yourself if your hard drive fails under these circumstances. Recovery
of files from a damaged hard drive is possible, but requires the
services of expensive specialists.
If you use a recorder with hard drive at high altitude, you can reduce
the risk of failure by running the recorder from a fixed position (a
table, a rock). This reduces the chance of physical shock to the
recorder while the disc is turning. Don't try to record while you're
walking.
Incidentally, the little hole on the bottom of the hard drive also
lets humidity into the interior of the drive. A hard drive's lifespan
is reduced if it's used regularly in a very humid climate.
There's little reason to use a recorder with hard drive any more.
There are many alternatives that use flash memory for storage,
including the $1900 Sound Devices 702, and the new Edirol R-44, a four
channel recorder that costs about $900. I can't think of a field
recorder that costs less than $800 and still uses a hard drive.
--oryoki
|