Why to never buy 3.5" external HDD's? A good number of reasons. But 2.5" are ok. If I haven't warned you guys & gals before then read up!
1- The drives that go into these external enclosures, like the WD MyBook series for example, are sub-standard.
2- More surface errors. Externals have more errors on the surface right out of the factory than their equivalent desktop models do. Every single HDD ever made in the entire world has surface defects. But externals have more than their desktop counterparts.
Every HDD comes with a bad sector map. The defective spots are recorded in a chart and the drive skips over them, substituting a bad sector with one from a reserve area. Thus the disk "presents" a 100% error free drive to the OS. And in the old days, you had to manually enter the defective sector list by hand according to certification chart taped to the drive at time of mfg. Today it is stored in th firmware.
Accessing these spare areas takes a few mSec more than a normal sequential sector - because the head has to swing to the reserve area. This latency (for external disks) is buried out of sight deep inside the USB interface. But on a hi-performace Enterprise class drive, speed loss due to defect remappings are noticable and thus they get the best platters.
3- Balance. You'll note that on average external drives generate more vibration. This is from sub-standard bearings in the motor/spindle and out of round platters. While each HDD (if you take it apart) seems ultra-precise, there's a real difference when looking at their tolerances on a microscopic scale.
4- Cheaper engineering. You'll find less cache in externals along with single processor cores as opposed to 2 and 4 core controllers. That, and number of heads. As the number of heads increase, the reliability decreases. The desktop equivalent will have less surfaces, thus less parts to fail, and fail they do!
5- Overall machining tolerances and materials are worse in the externals. The materials are more sensitive to temperature changes, parts expand and contract more, producing different geometries of the heads and surfaces, and therefore requiring more compensation from the electronics to keep things on track. Incidentally, this creates more heat. Typically 5-10`C. Might as well throw in +5`C more due to rougher and less than "perfect" machining of the bearings. And unbalanced platters require a bit more current to keep them spinning steadily. More stress more heat. Unbalanced platters add about 3`C to overall tempertature gain. All this consipires to compound things. The metallic coating (where your data is stored) has its coercivity affected by hotter temps, and presents a less defined magnetic pattern.
6- Larger parts, platters, actuator arms, motors, bearings, fly-height adjusters, micro-positioners, voice coils, things like that, it all takes more power to operate.
7- Lack of any form of cooling! That's correct. There is no cooling fan anywhere in most external disks! You're taking a desktop drive and basically putting it into a box. No air circulation. You don't do this with your desktop drives do you? You've got some kind of airflow, or if not, you've got the disk attached to a metal frame that absorbs some heat. In an external USB-style drive you don't any of those two cooling mechanisms. You might as well put the disk in a baggie and then a shoe box. This one factor alone is responsible for the high failure rate of external disks! Externals can typically run 20-30`C higher than their desktop cousins.
A hotter running disk from the get-go, trapped in a box with no ventilation, is just asking for trouble. And manufacturers know this! Evidenced by the shorter warranty 1-year vs. 3 or 5 years! They know the disks will fail, they know about the lack of cooling as the #1 reason. They are too cheap to put in a $5 fan, citing a number of excuses - one of which, ironically, is extra moving parts.
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I've seen Western Digital external drives run at a blistering 68`C !! How can you expect a cheap consumer grade product to operate reliably under those conditions. You can't! And I have to say that over the years I've had many drives both professionally and personally, and without a doubt, every single Western Digital 3.5" external disk that has no cooling fan has died prematurely. ALL OF THEM! 100% fail.
Manufacturers expect this. They expect a short lifespan. They know this. Remember that 1-year warranty as opposed to 3-years or 5-years!
It should be noted that 2.5" disks don't exhibit many of those cascading failure scenarios. They are engineered to work in an enclosed compartment with no ventilation. A laptop! They handle vibration better. Parts are smaller, better spec'd. Less stress, less power consumption. Better surface verification procedures. All a necessity with the higher density of 2.5" platters and portable environment.
So if you're looking for external USB disks buy only 2.5", afterall these smaller drives were engineered for the portable environment from the ground up. They were not afterthoughts and hey let's do this types of hacks.
And eventually, once SSD's become ready for the consumer market, then I will recommend those. For now, stick with 1TB 2.5" externals for your cost-effective storage needs.
Get two 1TB 2.5" disks as opposed to a single 2TB 3.5" external.