I know this is an oldish thread...
E: 2.1 TB
100GB: Primary boot partition for OS (partition on 500GB drive)
60GB: Old, functional backup boot partition for OS (partition on 500GB drive)
60GB: Experimental boot partition for OS (partition on 500GB drive)
1TB: Recorded TV (2x 500GB in Raid 0)
600GB: Online movie storage (2x300GB in Raid 0)
280GB: Videos, Music, Pictures & Downloads
All 3 bootable partitions use the C drive letter for the OS.
I use a stand alone 500GB drive (USB enclosure) to back up critical files & the OS on the HTPC, but I don't back up Recorded TV, nor Online movies (original DVDs are the backup).
Note that I haven't upgraded any of the hard drives in well over a year.
STB w/R5000HD USB I/O, Gigabyte GA-P35-DS4, Quad Q6600, 4.0 GB RAM, ATI HD 3870 512MB, Ultra XVS 600W PSU, 3x SATA 500GB, 2x SATA 300GB, LG GGC-H20L, PVR-250, Toshiba 51H83 (51" HDTV), Yamaha RX-V2400 Amp, 5x Energy Speakers, SVS Sub, Harmony 880 Remote