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Is there such a beast? Every USB-powered portable hard drive I've seen is 5400RPM (or slower), and every 7200RPM drive I've seen requires external power.

Looking for a small, fast, portable drive I can carry around with me, mostly for storing VM's. Any recommendations?

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closed as off topic by Iain Jul 28 '12 at 7:59

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7 Answers 7

up vote 5 down vote accepted

Before I get on to the actual question there is one point that I think might be quite significant here: in most cases, with modern drives, the data rate of the USB adaptor and the CPU hit associated with high-speed USB transfers are going to be more significant than the spin rate of the drives. There is no point spending power on spinning a disk faster to achieve a bulk transfer rate of 100MByte/sec when the USB transfer probably tops out somewhere between 25 and 30MByte/sec anyway - the faster spin, and therefore the extra power, is wasted.

The faster a drive spins the more power it will need, especially when initially spinning up form idle, and the standard says that a single port can only provide 2.5W of power (0.5A at 5V). For this reason you are unlikely to find 7200 RPM 2.5" drives in USB drives that do not require their own separate power adaptor.

Many "2.5" drive -> USB" adaptors have two USB plugs to allow drawing power from two sockets to pick up more power than the 2.5W one port can provide. Be careful with these and unpowered hubs though, as an unpowered hub is limited to 2.5W output in total *not( 2.5W per port (powered hubs do provide 2.5W per port, and many hubs can operate in both powered and unpowered modes). If I remember rightly USB3 will increase the range of power profiles available, including new higher-power options, but USB3 is a fair while away from general availability at this time.

The main advantage of spinning the disk faster is that more data passes by the drive heads in a given amount of time, increasing the bulk transfer speed. The same effect can be had by using drives with a greater data-per-track density (i.e. the same amount of data on fewer platters but the same number of tracks per platter) so you might find that there are disks which provide better bulk transfer speeds without increasing the spin rate. The other advantage of a faster spin rate (slightly reduced latency for random access) can not be helped by greater data density (though this latency is small compared to that imposed by the physical head positioning time).

Before plugging any drive into a USB adaptor that does not have its own power, check the power requirements of the drive you intend to use. Many drives, especially those that spin faster or are just older, may want to draw more then 2.5W when spinning up even if they need less during active use once powered on. For instance looking up details for the drive I have sat on my shelf here (a 20Gb drive pulled from a dead laptop) it is rated as needing up to 2.9W when spinning up (see http://sdd.toshiba.com/techdocs/MK2016GAPuserguide.pdf) so would not be suitable for plugging into a single USB port.

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I have a Buffalo 250 GB self-powered (e.g. with its own power supply and cord) external drive, I don't know what speed it is, but I run it off an unpowered hub just fine, and it seems about as fast as the internal drive on this machine. This makes me suspect what other people have been saying, once you get to 5400 RPM it's pointless to worry about additional speed as you're saturating a USB 2.0 connection anyway.

Paul Robinson paul@paul-robinson.us

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I'd recommend the Western Digital My Passport (5400 rpm) external drives, which I use for running VM's off. Performance is great. I used to use Western Digital My Book (7200 rpm) drives but got a bit tired of having to lug it about (due it's large case) and also with it needing an external power supply.

Comparing the two types of drive I've used, I've noticed little difference in speed when running VM's. I'd stick with a 5400 rpm drive.

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My recommendation is to stick with the 5400 RPM ones - the 5 Watts from the USB might not suffice for a proper 7200 spinner, and USB 2 is capped to 40-45 MB/s, which modern 5400 RPM drive should easily saturate.

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A single USB2 port can only be expected top provide 2.5W (5 volts at 0.5 amps), not 5. This is why many "2.5" IDE/SATA -> USB" adaptors have two USB plugs, to draw current from both for a total above 2.5W. Be careful though - the total current that can be provided by an unpowered hub is also 2.5W so plugging both into an unpowered hub will not work well. –  David Spillett Jul 12 '09 at 12:17

You can get yourself an enclosure and a 7200 rpm 2.5 inch laptop drive drive. I've never seen any specifications of the usb enclosures that say they do not support 7200 rpm drives. Alternatively you might want to put in a fast SSD. Granted price/size wont be as good but if your running the vms off the storage the high read speed would be beneficial.

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If you want fast, I'd use eSATA instead. Carry a PCMCIA eSATA card with you. too. It doesn't meet your requirement for no external power, however.

Actually, when I looked just now, I found one that has the transformer in the case so you don't need a wall wart.

SATA Enclosure

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i have one with an external caddy that i use and it is just powered by usb. i think the case i use to put it together was from vantec. the hdd was a 500gb samsung and i also did it with seagate one too.gd

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