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I am considering two drives for a backup drive.

1) 1.5 TB WESTERN DIGITAL Elements Desktop WDBAAU0015HBK: 3.5", USB 2.0, 7200 rpm, 32 MB cache

2) 1 TB WESTERN DIGITAL My Book Essential WDBACW0010HBK-EESN: 3.5", USB 3.0, 5400 rpm, 64 MB cache

Since I can only use USB 2.0, two relevant factors are rotation and cache. Which one has a higher impact on performance, cache or rotation? Would a 7200RPM/32MB drive perform better, or a 5400RPM/64MB drive

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Disk cache size has almost no effect on performance. The cache just needs to be large enough to adapt the rates and latencies of the two interfaces it bridges. Every IDE drive made in the last decade has a large enough cache to do that. –  David Schwartz Feb 15 '12 at 18:06

2 Answers 2

up vote 3 down vote accepted

For sustained speeds, ie: not bursts. Rotation speed is going to make a larger difference. Considering your limitations are USB 2.0 you're unlikely to see a saturation of the drive though. From Wikipedia:

USB 2.0: Released in April 2000. Added higher maximum bandwidth of 480 Mbit/s (60 MB/s) (now called "Hi-Speed"). Further modifications to the USB specification have been done via Engineering Change Notices (ECN). The most important of these ECNs are included into the USB 2.0 specification package available from USB.org

You'll likely never hit 60MB/s via USB 2.0, and both of those drives via their spec can hit that. So the primary limiting factor here is the USB interface, not the drives.

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What about if I add USB 3.0 slot. What would then be the difference between these two drives? –  John Feb 15 '12 at 17:23
    
USB 3.0 is 625 MB/s, so the limitation will now be the drive. What I said earlier is still true, for sustained speeds the higher rotational speed will provide better transfer speeds. Drive cache makes the largest difference on small (ie: near the size of cache) writes and sequential reads. If you plan on reading/writing the same small dataset (~64MB) frequently, go with the larger cache size, otherwise the rotational speed will make an overall larger difference. –  jidar Feb 15 '12 at 17:33
    
Ok, so why would some make a USB 3.0 disk and drop the rpm to 5400? Does it make sense to buy a USB 3.0/5400 rpm disk? –  John Feb 15 '12 at 17:46
    
Because both disks will saturate a USB 2.0 Bus, so yes it makes sense to buy either as a USB 3.0 device. It's up to you if it makes sense. Does it cost more for a 7200 RPM disk? Is the cost justifiable or not? I cannot answer these questions. I can tell you that 7200 RPM Disks net around 75 (on average) IOs per second, or disk operations. 5400 RPM disks net less than that. 7200 RPM Disks also consume more power (another marginal difference). –  jidar Feb 15 '12 at 18:00
    
I just learned that 3.5 disks actually demand an external power source, compared to fact that 2.5 disks only require USB connection. This is a big advantage for 2.5 disks in my opinion –  John Feb 16 '12 at 7:07

The speed of the USB port is going to be you're biggest limiting factor here. I'm not sure you would see a noticeable difference between a 7200 RPM and 5400 RPM USB connected drive. This of course, is a mix of opinion and "what I've noticed". Someone may come along with a more enlightened answer.

That being said, is eSATA an option for you?

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Actually I don't have FireWire 400 connection. What did you have in mind if that was an option? –  John Feb 15 '12 at 17:24

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