Having all SSDs is the fastest approach, but obviously that would be expensive. With only some SSD drives, it's actually better to have your OS on the SSD, even if you have to store videos and work with videos on HDDs. With on OS SSD, Windows boots in around 10 seconds and programs all launch very quickly. For example, PD12 opens in 2 sec on my machine. Once you've seen responsiveness like that, there's no going back
I have 3 SSDs on my new machine: a 120GB OS drive and I put my current videos on a 240GB and 1TB SSD drives. I also have about 6.5TB on 2 HDDs as archive storage. To be honest, I don't see a whole lot of difference when opening projects with videos stored on SSD vs. HDD, and there's no difference in playing them back because HDDs can handle the 28-35Mbps video stream.
A couple other thoughts: If you use HDDs, be sure to use 7200RPM drives instead of the older (and much less expensive) 5400rpm. The transfer speeds are noticeably slower with the older ones (R/W on a 7200RPM dive = 78MB/s and 154MB/s; 5400RPM = 31MB/s and 72MB/s on my system). Note that the write speeds with hard discs are much higher because the drives have a large cache memory which can accept data about twice as fast as it can be written to the disk. The read speed is basically how fast the data can be read off the spinning platter, which is why faster platters = higher transfer speeds.
There's a similar divide in SSDs, where the least expensive ones can read at about 500MB/s but can only write at 200MB/s. My first OS SSD drive was like that on Win 7 and I thought that read speed would be the most important factor, but after using a higher-write-speed SSD I can say it's definitely worth the extra cost to get the symmetrical 500MB/s R/W speed. I really like the Samsung 840 EVOs, but anything you can find with those kinds of specs would be worthwhile.
EDIT: I changed the drive transfer speed references from Megabits per second (Mbps) to the correct Megabytes per second (MB/s), as the rate is how fast each byte (word) of data can be written or retrieved, not the speed at which the SATA serial interface runs At least I had the video bitstream rates correct!
This message was edited 1 time. Last update was at Mar 17. 2014 15:35
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