I'm using three harddisks in my system the first one (IDE) is only for the boot system, the second for data (documents, pictures, disk images, temporary files, downloads and whatever), and the third contains streming media like movies, DVD images and my iTunes music files.

My boot HD is splitted into three partitions: The main partition, A recovery partition which contains a second bootable system and a Shared partition which is used for .dmg backups and other system data. You can read more about this here.

I don't see any sense in wasting a whole harddisk only for a Time Machine backup which may be useful in case of an important data loss, and this isn't happening every day. In my case I only have to backup the home/library folder sometimes and my iTunes database, I wouldn't loose more data even if a complete reinstallation would be neccessary. And this is also nearly unneccessary because I can repair my main partition always with my recovery partition.

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2 Opteron systems: OSx86 10.5.8, Andy's 9.8.0 kernel, Asus A8N-SLI Premium, Opteron 185 o'clocked @ 2 x 2,95 GHz (2nd system 2.6 GHz), ATI Radeon HD2600XT 256MB Dual-Monitor 2x HP L2035, 4 GB RAM, Griffin FireWave as main audio device, Marvell + nForce LAN, Asus U3S6 USB3/SATA6 card, 5,5 TB harddisk, Firewire 800 card, Apple Remote + eHome IR receiver, 2x Wacom serial graphics tablet, Canon Pixma iP4700, Logitech Internet Navigator wireless keyboard/mouse combination.

My Audio stuff: M-Audio Transit USB (default audio), M-Audio ProFire 610, M-Audio ProFire Lightbridge (34 channels) using Creamware A16 ADAT converter • MIDI: M-Audio Midiman 4x MIDI interface • Behringer Audio Mixers: Xenyx 1002, Xenyx 1002FX, Xenyx 1202FX, Eurorack UB1002FX, Eurorack MX1804FX, Eurorack MX262A • FX devices: Lexicon MPX100 DSP, Behringer DSP-1000 Virtualizer, Behringer MiniFEX 800 DSP, Behringer Multicom Pro MDX4400 compressor • RETRO: MSSIAH midi/sequencer/synthesizer cardridge for the C64 (Dual-SID), Steinberg M.S.I. MIDI Interface for C64