Three. The internal. a 320gb for movies/tv shows and Time Machine, and a separate 80gb that is just a backup of my iTunes library.
I'm assuming iPhones and thumb drives don't count, right?
The ones I don't use, but have because I'm a packrat-
A 40gb from an old Acer laptop in a firewire enclosure, an 80gb from my old G4 iMac. Has a bunch of files I have a feeling I'll want someday. I should probably buy an enclosure to get to them while I can.
Oh, and an 80gb in a Sony Vaio that was Hackintoshed, and is now bricked. Should part it out and dump it on eBay.
3. 160GB internal. 250GB on USB with photos, videos, and archived client data on it (I keep music and current client files on the internal so the external doesn't have to be running constantly). 500GB on Firewire partitioned into a) backup of internal drive, b) backup of external drive, c) Photoshop scratch.
I have 6 HDDs:
160 GB internal (MacBook)
320 GB external FW (General scratch disk, downloads, and TM backup)
500 GB external FW (Backup disk image, Leopard install, HD Movies)
500 GB external FW (Tiger boot, non-HD Movies)
500 GB internal bare USB (TV shows)
1.5 TB internal bare USB (Consolidation of HD Movies, Movies, and random funny clips/commercials)
The 1.5 is redundant to the other two drives that have my movies, as it is new I don't trust it enough to use the other two drives for other things. All except the MacBook drive are 3.5" and the two bare drives are connected using a USB to SATA cable and dock.
Another question: What is the minimum number of drives to own if you intend to keep data backed of and safe "forever"?
I'd argue the answer is "four"
A widly accepted rule of thumb for data is that
1) The data must always exist on three different physical media and
2) The data must always exist in at least two different geographical locations
"Always" means even durring a backup operation. So four is the smallest number. But many of us have more data than will fit on one drive so we'd need many more drives.
Hardly anyone actually does this so I've many this prediction many times: "in 100 years there will be very, very few 100 year old photos."
I, like everyone else, have a primary internal drive in every computer you can see in my signature. Beyond that I have three externals and the MDD home server/storage is loaded up with four 1tb drives for various storage and backup options. I also have a portable 60gb drive from where I upgraded the stock drive in my iBook. So, lets see, if you include all the internals then there's a total of 13 hard drives in my house. Wait, I forgot iPods. With three iPods in the home that brings the drive total to 16. Drat! forgot about my DVR and PS3. Okay, 18, yep that's it. There are 18 hard drives used regularly in my home.