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Influence of Drive Speed on CS5 Big File Save Speed
Up to a point, a faster volume helps, assuming one is using the DisallowFlateCompressedPSD.plugin.
A volume is not a drive; a volume can be a single drive, a partition on a single drive, or a RAID-0 stripe of partitions across multiple drives. The physical drive(s) could also be solid state or hard drives.
A 3-drive RAID-0 hard drive stripe capable of ~435MB/sec is about where Photoshop CS5 overhead kicks in so that further gains are minimal.
With the advent of 500 MB/sec speeds for fast 6G solid state drives, a single SSD will provide most of the raw drive speed for most users in any Mac that supports 6G SATA III. However, difficult-to-compress data slows down the Sandforce-based SSDs, so a dual-SSD RAID-0 stripe is a better bet for maximizing speed.
The measured sustained transfer speeds here for 1/2/3/4 drives were 150, 302, 435, 580 MB/sec, using the Hitachi 7K3000 3TB hard drives inside a 2010 Mac Pro 3.33 GHz 6-core. Get Disktester if you want to measure your own drives.