More platters + more heads equals higher chance of failure.
Take two common WD hard drives
640GB = two platters
1TB = three platters
WD Black 640GB vs 1TB comparison
Drive Ready Time 11 sec 13
R/W Power watts 8.3 8.4
Idle Power watts 7.7 7.8
Standby watts 1 1
Max shock 300g 250g
Performance seek 29 33
Quiet seek 26 29
That extra platter = more noise, more power usage, more heat, slower drive ready time, more susceptible to shock damage, and more vibration.
If they made the same drive design with only one platter it would have even better specs. In this case these are consumer grade drives but they are high end consumer grade drives with double the cache and a 5 year warranty. You'll see similar math if you closely inspect the documentation on any brand or style of traditional hard drive (spinning platters). It's purely a matter of physics that more platters makes a drive less reliable.
Jeff Hengesbach was also right when he said
The primary concern with 'big' drives
is the rebuild time when a failure
occurs. The larger the drive, the
longer the rebuild, the larger the
window for additional drive failure
and potential loss of the array. With
"big" drives the business value of
availability should determine a level
of acceptable risk(array loss) which
will drive your RAID level selection
and drive count(More drives = more
chances of drive failure).
add in some small dose of Graeme Perrow
A drive with fifty million sectors has
ten times the chance of having a bad
sector than a drive with five million
sectors. I'm assuming the failure rate
among large drives and small drives is
the same here, which is probably not a
More platters = bad
More storage space is a mixed bag. Pros and Cons on that are numerous.
More sectors really is more chance for errors. Not necessarily linear in scale but definitely a factor.
Unless you need space more than reliability I would suggest sticking to single platter or dual platter drives. It takes research and in some cases luck to know what you'll get when ordering drives as some manufacturers not only avoid publishing the number of platters they may actually sell more than one drive under the same part number.
Take for example the WD3200AAKS there is a single platter 320GB version and a dual platter 320GB version (160GB x 2). On top of that there are multiple lables and drive housings being used so you can't easily look at the drive and know which platter is inside. The only way to know is to search online to know that WD3200AAKS-00B3A0 and WD3200AAKS-75VYA0 tell you which is single platter but no retailer will tell you which you'll get.