iMac: 500 GB,
External USB HD: 500 GB,
Two 160 GB USB HDs on my AirPort Extreme Base: 320 GB,
Three iPods, a total of 8 GB,
Two PCs, a total of 240 GB,
Two laptops, a total of 120 GB.
All of the drives listed still work, I just disconnect them when they get full, or migrate them to larger drives. I don't like having too many external drives connected. Too many things to have plugged in, and I don't like waiting for all of them to wake up when waking the system, or waiting for them to wake up when they've gone to sleep while using the system.