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3 x 1TB LaCies and one 500GB Philips. Two of the LaCies are for storing my movies, TV shows, music etc, one is for Time Machine and the Philips one is for storing all kinda crap and Windows backups etc.
 
(1) Drobo w/ 3 1tb drives. I use this for iTunes and backups of any AVCHD media I now capture, plus as storage for some old video footage.

(2) 2TB OWC Raid 0 Mercury Drive - Used for Video & Audio Editing (Final Cut Pro & Logic)

(3) Time Capsule - Backup of my iMac, which has iPhoto, iMovie and all my documents on it.
 
Vantec Nexstar 3 Black Aluminum 3.5IN SATA Hard Drive Enclosure eSATA USB2.0 Firewire IEEE1394

WD 500GB Hard Drive

I bought two of these setups as we have two Mid 2007 Imac's

*The Vantec works great for booting (firewire) into the External Drive, We use Super Duper Backup Software. It really came in handy when our second iMac HD's Died just like the first one did. Grrrrrr! :(
 
1 drobo 2nd gen with 2x 1TB drives for all of my main storage for photos, music, videos, and documents
1 generic enclosure with a 1TB drive for time machine
 
I've just got a 1TB Seagate Freeagent Desk for Mac. It seems to be working perfectly with Time Machine so far.

Before the Seagate, I bought a 1TB WD MyBook Studio, but had to take it back, as it was making my iMac unstable after sleep, and it would randomly miss out files from my Time Machine backup.
 
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