Originally Posted by JH4music
You need to use a 2.5" SATA drive(Notebook size). You also need a special caddy to put the hard drive in as the SATA optical drives have a different connector. What makes the most sense is a solid state drive, since mechanical drives in the notebook size are never as fast as their 3.5" big brothers. You also need a set of suction cups to pull the glass off. This is not for the faint of heart, but if you're used to working on laptops it's not that much different.
ifixit has a complete teardown tutorial: iMac Intel 27" Teardown - iFixit
You can also buy the optical to HDD adaptor from them: 12.7 mm SATA Optical Bay SATA Hard Drive Enclosure - iFixit
I put an 80GB intel X-18M SSD in mine, and use it as a boot drive, I use the 1TB drive for audio and storage. The performance increase from the SSD is amazing. My imac boots in 5-10 seconds, and applications and sessions load ridiculously fast.
Well that certainly sounds orsm
I will do something like that down the track for my i7 iMac when I start pushing it's limits, It's great the have everything in a tidy iMac package, when mixing ITB I like to have as much ITB as possible