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The question I'm asking is kind of general, and I'll give more specifics about my specific setup. Here's the main question though: Do virtual machines generally perform better on the host HDD or is it better to operate them from an external disk?

My specific setup: A Macbook Pro with a nearly full internal SATA drive that spins at 7200. On this system I'm running large programs like Photoshop and some other RAM-intense applications. I've dedicated 2 of my 8 gigs of RAM to my VMware Fusion virtual machine, which runs Windows 7 and Visual Studio, sits on the same drive. When that thing boots up, my system really starts crawling. I have an external USB (specifics of that drive are here) which I'm thinking about moving the VM to.

Obviously a USB drive is slower than my internal HDD, but maybe having two operating systems using the same disk is WORSE than putting one of them on a separate (albiet slower) disk.

This a bad idea?

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up vote 1 down vote accepted

If speed is what you want, running off an external drive connected via USB is not the answer. Access to the USB drive is going to be slower than to the internal drive; SATA offers several times as much bandwidth as USB.

A nearly full drive slows down writes due to the difficulty of finding free blocks and the fact that they are spread all over the drive. You will be better off in terms of speed freeing up space on the internal drive by moving less used files to the external drive and continuing to run the VM off the internal drive.

But, and this is a big "but", you have not as far as I can see identified the bottleneck that's making your system run slowly. If you're simply running out of CPU or your system is swapping because main memory is full of active pages, moving files around on your disks isn't going to help. Run Activity Monitor on your Mac and check system memory and CPU usage. Also look for page out activity, 0 bytes per second is what you want to see.

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Yep, bad idea. USB drive speed is lower than the speed of SATA drive in your computer. However it can be usable even with this scenario.

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Updated for clarity. "Obviously a USB drive is slower than my internal HDD, but maybe having two operating systems using the same disk is WORSE than putting one of them on a separate (albiet slower) disk." –  Jeremy Ricketts Mar 30 '12 at 21:44
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A very bad idea: the speed of the USB port is much slower than the internal hard drive. While it will work, you will lose performance.

Here you can see charts of average read speed of external hard drives, and the USB ones are around 30 MB/s, while an internal SATA HDD should be able to deliver at least twice that speed.

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It's not such a bad idea that it isn't worth a try. (Or to collapse the negatives: try it.) People boot off external drives, and such setups are not grossly unusable. FireWire is better than USB 2, though.

The idea of utilizing all available drive "spindles" has merit. Throughput is not the only factor; you're suffering from latency with all the disk thrashing.

And with VMs, it's very simple to move the virtual drive to the USB.

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