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I am surprised to find that all portable external HDD of size >= 1TB today are 5400 rpm instead of higher 7200 rpm.

I wonder what is more important for portable external HDD, interface (USB 2.0, USB 3.0, firewire, etc) or rotational speed (5400 rpm, 7200 rpm, etc). Thanks!

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What is most important is how you are going to use the external drive.

If you use it to store media files such as movies then you just read and write very large files. Reading these is usually a lot faster then you can display a movie. This means low noise and possibly low power are important. Even an old 5400RPM drive will suffice qua speed. And a low RPM drive is likely to make less noise than most other drives.

If you use it to store a large number of small files, randomly accessed then it is a totally different story. Access time gets very important. That depends on how fast your disk platter spin. (A drive only reads data below it read/write head. If you want to read something just head of the head than you are lucky and you will have your data in almost no time. If the data has just passed the head then you will need to wait until the disk has spun almost a full circle. With multiple accesses this balances out and you will have to wait an average of half a disk rotation for new data. Faster rotations help.

I wonder what is more important for portable external HDD, interface (USB 2.0, USB 3.0, firewire, etc)

The interface also depends, but.

  • USB1. Ancient. SLOW. Do not use. (Copying a 40GB disk takes a full weekend)
  • USB2. Universally used. Not very fast. Has significant overhead in the protocol. Not easy to access unless your OS has drivers for it (which any modern OS has. But still anoying when booting rescue disks). It only real advantage is that every PC has an USB interface.
  • USB3. New. Found only on a few new computers. A lot faster. Interface speed is good enough, though it has the same other disadvantages of USB2 (protocol overhead, drivers).
  • Firewire: Found on only half of modern PCs and laptops. Usually somewhat faster than USB2. Has some nice advantages over USB when streaming multimedia files, but for HDD access these are not important. A firewire interface is found on a lot on professional euipment, but due to royalties only half of modern PCs seem to have this interface.
  • eSATA. Often as fast as the drive supports. No additional overhead. Works always. Technically the best interface for a HDD since it's the native interface of the drive. Like USB3 it is not present on all computers. eSATA does not need any additional drivers. Ever. You can just remove the drive from a existing computer, plug it in externally and boot it without problems and without modifications. You can boot ancient DOS and access the drive. I think every professional IT person loves it. As do people who need pure performance.
  • Thunderbolt: Not really a drive interface. Think of it as PCIe via a cable. You can plug a SATA card in your PC and plug a drive into that, or but all of that behind a cable and archive the same. It will not work better then eSATA, but the interface can also be used for other stuff (e.g. external GPU, external RAID (incl. HW RAID card), serial interfaces, USB interfaces, you name it.). This means that while for pure HDD access it is second to eSATA, it is a lot more functional. This makes it a good choice if you do not want a dozen different connectors but still want very high speed.

ll of these interfaces have a published speed. You will never reach that speed for data transfer. The published speeds are usually the 'wire speed', or the speed on which signals can be sent across. The makes sense of these signals you will need to decode them. Usually this causes about 25% overhead. eSATA is the only option with the least unnecessary overhead. It is also the least flexible (e.g. you can use to to communicate with a HDD or a CD/DVD/BD, but you can not talk to a camera or keyboard or … )

or rotational speed (5400 rpm, 7200 rpm, etc).

For pure sequential access (read: large files on a defragmented filesystem) almost any drive speed will do. The faster the rotation speed the faster the drive can read data, but unless you use a very fast interface that speed will be capped at the speed of the (slower) interface. In which case the difference is not significant.

For mostly random access: Get the fastest rotation you can afford, or get a SSD.

If you use eSATA or thunderbolt: Get a SSD. In these cases your interface will not slow you down.

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"Thunderbolt: Not really a drive interface." - Well neither are USB and Firewire. All of these are busses that can be thought of "as PCIe via a cable". –  sawdust Aug 13 '12 at 3:25
    
True. All three are busses. I will add that to the answer. –  Hennes Aug 13 '12 at 11:08
    
+1 for the first comment: "how you are going to use it"? Making a backup is another case where rotation speed is fairly irrelevant. Sometimes there really is such a thing as "fast enough". –  MSalters Aug 13 '12 at 12:32
    
Thanks! (1) What do you mean by "protocol overhead" for USB 2.0 and 3.0? (2) For firewire, why is it "for HDD access these are not important"? –  Tim Aug 14 '12 at 14:57
    
@MSalters: " Making a backup is another case where rotation speed is fairly irrelevant." Is it because it is sequential reading and writing? –  Tim Aug 14 '12 at 18:07

As drive capacity increases, rotational speed is less important due to the increasing drive data density.

i.e: more data in the same amount of space means more data goes under the read head at the same speed.

That said, i'm not certain where the exact cross over point is in terms of when interface becomes more important, but given that USB 2.0 is much slower than the physical drive behind it, i'd say the interface is more important than the speed. As long as we're not talking about a drive at 1 rpm.

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I agree, I know just from "feel" that a file copy from an internally connected 5400 RPM drive is a LOT faster than 25 MB/sec which is about the sustained throughput of a USB 2.0 connection. Firewire is faster still (though spec MAX is LESS, sustained throughput is usually 15%(+/-) faster) but only relatively modestly faster. Connection via USB 3 is up to 5 Gbit and if I'm correctly doing the math in my head, that, and definitely SAS/SATA/eSATA/IDE/SCSI are all faster than a 5400 RPM drive's speeds are typically able to produce, so if your connecting like that, you want to use faster RPM. –  Multiverse IT Aug 12 '12 at 20:25
    
Yes, for low data capacity drives. Don't get me wrong, a 2TB 15k drive would beat a 2TB 5k drive, but a 2TB 5k drive will beat an 80GB 15k drive (for arguments sake, citation needed really). As long as your interface isn't the bottleneck then yes, rotational speed matters. These days though as drive capacities have shot up exponentially, speed gains have been made more through density increases than rotational ones. –  Sirex Aug 12 '12 at 20:33
    
Don't forget also slower drives use less energy, last longer, are less expensive and easier to make, etc. All good additional bonuses. I should prolly also mention that drive speed is not constant across the drive. The outside edge spins at a higher rotational velocity and hence has the fastest performance. Some people put database and swap files there for that reason. –  Sirex Aug 12 '12 at 20:34
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Modern large capacity drives also often have very large internal buffers, which also makes rotational speed less important. –  martineau Aug 12 '12 at 22:15
    
Agreed for sequential access. (But not for semi random. But most external drives seem to contain movies, music etc and that makes your comment relevant for most users). –  Hennes Aug 13 '12 at 0:10

Let just say this, new 10000 RPM drives dominate with the speed (they even have their own heatsinks). Use eSATA for external hdd's.

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USB3.0 transfers data at ~50MB/s, whereas USB 2 at up to ~5MB/s. Firewire transfers data at similar rate as USB3. That is a factor of 10x. The next generation Firewire from Apple, called THUNDERBOLT, has transfer rate of 1.5GB/s, almost fiberoptic category.

7200 rpm drives are better for PC than notebook as they require more power.

That said, a 7200rpm drive transfers data (depending on other things equal, such as, bandwidth, RAM size, whether DMA is used or not, etc..>) roughly 33% faster. However, faster drive tend to fail mechanically, especially when the drives are not routinely (twice a week) defragmented.

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Your numbers are off. USB 2.0 has a throughput of 480Mb/s (60MB/s) and usb has a throughput of 5Gbps (625MB/s). Thunderbolt is 10G (1250MB/s). SATA II is 6GB, so as you can see usb 2.0 severely bottlenecks it. (actual drive throughput will be lower, this is purely on specification stats). –  Sirex Aug 12 '12 at 22:53
    
Also fwiw fibre optic is a type of cable, not a single specification or cable. There's fibre optic cables that run at 10Mb speeds, and some that run in the hundreds of Gb/sec –  Sirex Aug 12 '12 at 23:01
    
1) Isn't USB2 480mbit - 10% reserved for USB1 (thus leaving max 432Mbit raw line speed for the USB2 part. Divide by 10 for average overhead. That would make it max 43MB/sec. 2) SATA II is not 6GB. SATA II is 3.0Gb/sec. SATA III is 6.0Gbit/sec. –  Hennes Aug 12 '12 at 23:24
    
Yes, indeed re: sata III. typo. - also just noticed "and usb has" should read "and usb 3.0 has" –  Sirex Aug 13 '12 at 0:29
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-1. There's quite a bit wrong with this answer, but the "fail mechaniaclly when not defragmented" is probably best. The glass platter itself really doesn't care whether sector #78820 contains the same file as sector #78821. In fact, the disk itself knows nothing about files. –  MSalters Aug 13 '12 at 12:35

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