Dell offers a laptop with either a 500GB drive or two 320GB drives (both 7200rpm and no options for SSD). I will be running VMs for development and will also have several TrueCrypt mounts.
I figure two drives are better for a): faster disk access, and b): data is on separate drive from OS. Is that right?
Scott Hansellman made a blog post with great advices about VM performance. Jeff Atwood too, with some benchmarks (it's from 2006, but still useful). And all of them agree about a second hard drive for your VMs. And I agree with them. When I use VM with a virtual disk in the same disk in my Vista it's a pain.
I would follow the advices presented in these links, since they are from programmers too, so you may have similar problems.
That would be a good solution. Keep the VMs on one and the OS and other stuff on the other. We found a significant improvement running the VMs on external eSATA drives for systems that did not support two drives.
Don't worry about performance - two disks are only better when you're working with them both at the same time, such as when copying from one to the other.
Or if you have two VMs active and working at the same time, having each on its own disk would improve disk throughput, on condition of having a multi-core (or multi-thread) CPU. But if both active VMs are on the same hard disk, then it's totally unimportant on which disk they are.
That said, here's some mathematics: 2 x 320 = 640 > 500. So you gain 140 GB.
If you go for the 2 disk solution, for me 320GB is too large for a system disk. I would go even further and partition the first disk into, say, 40 GB system disk with Windows and all the applications. This make it easier to backup an image of the system disk, which I do systematically just-in-case before I let Microsoft's Windows Update destroy my running system.
In that case I would have:
C = disk 1 partition 1 (40 GB)
D = disk2 (320 GB)
E = disk1 partition 2 (280 GB)
Please note that repartitioning the C drive will probably involve reinstallation of Windows (or restore from the OEM restore partition after re-partitioning).
It really depends what you are going to be virtualising.
I previously said in another post that for example, I can have around 10 VMs open at any one time without noticing any sort of slow downs when I am doing various tests on low spec machines that are not I/O intensive, where as, if I am doing anything a little bit intensive - just having 2/3 open can bring the computer to a stand still.
If you are planning on anything intensive or want loads open at the same time, get a second hard drive - otherwise, you should be ok.
If the second drive is faster than the first, and you are able to have unfragmented volumes on your second drive, VM will be faster. If you're first internal drive is faster and well defragged, that one will probably perform better.
