Solid State Disks (SSDs) are all the rage these days. Conventional hard disks read and write magnetic patterns on spinning metal platters - a bit like a high-tech a compact disc, but using magnets rather than a laser. SSDs, in contrast, use flash memory. Flash is more like the Dynamic Random Access Memory (DRAM) computers use as working space to load and run programs, with one critical difference: RAM loses its contents when the power is turned off, but flash memory retains its contents even without power. Flash is also what USB 'thumb' drives and camera memory cards (SD/SDHC, Compact Flash, etc.) use.
Flash is slower than DRAM, but faster than
This makes flash memory a viable substitute for spinning hard disks. SSDs offer several advantages: with no moving parts, they are more durable and less likely to break. Since there is no motor to spin the platters at high speed, they use less electricity - particularly important for laptops running off batteries. But most important, flash has no "seek time". Since every bit of information is stored on a particular area of a hard disk, reading or writing a piece of data from a hard disk requires that bit to pass under the read/write arm -- this is called 'seeking'. If the desired location just passed under the arm, this means the hard disk (HD) must wait for a full rotation of the disk before it can access the data there. Hard drives run at a variety of different rates -- from 4,200 to 15,000 RPM -- but reading data involves a great deal of waiting during seeks. For predictable patterns and large files, the operating system and hard disk can often lay out information in a contiguous strip, so the disk can read/write it as quickly as the disk spins, but running different programs which access different files at different locations can take a substantial toll on performance. This is why vendors, including Apple, charge more for faster (and more power hungry) hard drives.
But SSDs eschew moving parts and seek times. The computer just specifies an address in memory, and the SSD reads or writes it without pause. Of course, there are downsides: SSDs are subject to fragmentation, although it's somewhat different than hard disk fragmentation. And they have a limited lifespan -- after many writes, SSDs can no longer change the data they contain, although flash vendors work around this by adding intelligence to their controllers to spread out writes and compensate for errors, providing a lifespan comparable to a spinning hard disk. But most important, SSDs are newer and more expensive -- which generally means they cost more than hard disks, for lower capacity.
Most SSDs are sold in 2.5" hard drive style cases, with SAS hard drive style connectors - intended to replace hard drives in standard bays. This is how Apple sells SSDs in the MacBook Pro, for instance -- the SSD option is a direct replacement for the HD option, and available from many third parties. The Mac Pro uses SSDs packaged in 3.5" hard disk style carriers to fit the Mac Pro's drive bays, but the principle is the same.
The iMac, however, only has one hard drive bay, so Apple decided to use extra space in the 27" model for an SSD bay. Similarly, the MacBook Air doesn't have a drive bay -- the SSD is built onto the motherboard, saving space and reducing weight.
This gives 27" iMac and Mac Pro users an interesting 'hybrid' option: to run off both an SSD and a hard disk. Apple installs Mac OS X on the SSD, if present -- leaving any additional hard disks blank for user files. Unfortunately, when I researched and eventually received my new iMac with both SSD and SATA hard drive, I found some gotchas which complicated taking full advantage of both drives.
First, Apple does not include the mounting bracket in iMacs sold without the Apple SSD. Third parties do sell (rather complicated) rigs to attach non-Apple SSDs inside the iMac, but installation requires removing removing the monitor from the front of the iMac, so I decided not to try this, and instead ordered the iMac with Apple SSD.
Additionally, despite their similarities, SSDs and hard disks are quite different internally, so taking full advantage of both requires consideration of what files will work best on the SSD versus on the hard disk. Clearly large sets of files won't fit on the SSD - so I put my iTunes music & videos, my iPhoto Library folder, and my iMovie files on my SATA hard disk. Unfortunately,