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I'm running Windows 7 through Parallels 6 on a 15" Macbook Pro (2.53 i5, 8 GB ram) with the Momentus XT 7200 RPM 500 GB drive (it has the SSD hybrid). Would it run faster and give me better throughput on my Mac if I ran the VM off an external USB 2 drive?

I've read that if I had an internal SSD it wouldn't matter because there is no moving parts, but is it still worth it without an SSD?

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The internal drive uses a SATA connection, probably SATA2 with a 3Gb/s transfer rate. USB 2.0 has a transfer rate of 480Mb/s

So no matter how fast the disk is, it will always be slower on a USB 2.0 connection than a SATA connection.

However, running the VM off a different disk to the main OS will positively impact performance of the main OS. How much impact is dependent on the behaviour - if the Windows VM is doing a lot of IO and blocking the main OS, then you may see some performance benefits. Usually the impact to CPU and memory is far greater.

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