It's a LOT slower, lol. I use a RAID 0 of OCZ Vertex 30GB SSDs to get 60GB (plenty for a C: partition). I do so because of performance. But the WD Black 1TB drives are amazing values for their performance level when compared with other mechanical drives. I actually run one as my D: and E: partitions to store my documents, media files, etc. They are solid for massive amounts of storage.
You're just not expanding your mind enough to consider all of the possibilities.
I'm only using 134GB on my 1TB drive right now, but I can easily use more. Some examples:
- On my gaming rig I sometimes use FRAPS to record video of a game I'm playing. In raw format at 50% size of my 3840x1024 resolution (3x 1280x1024 monitors), which is 1920x512, it eats up 1GB every minute. So an hour long recording is 60GB.
- On my Ubuntu Linux system running a program called "MythTV", I have it scheduling and recording TV programs (it's acting as an HTPC). It takes 7GB per hour of recording in a very high quality format. It only has a 250GB hard drive, and split into two partitions, with one for the OS and the other for the sotrage (197GB for recordings), I'm limited to recording and saving 28 hours of TV at one time. I'll be switching this older system out for my previous gaming rig, so I'll upgrade to a WD Black 1TB drive at that point.
- My relatively small MP3 collection is 20GB. I can easily ramp this up to 100GB or so now with tons of space. I put off collecting too much in MP3 format in the past because of drive limitations. Now I don't have to hold back.

- All of my real documents are 89GB. This includes images I work on in Photoshop, Office documents, resumes, my programming projects, and other small projects, etc.
And I wouldn't dare run all of these things on an external drive. That's just silly since I have plenty of room in my case and my case has better cooling than an external enclosure for a drive. The performance is better (unless you run eSATA for your external drives), there are less cables to deal with on your desk, and your drive is better protected in a PC case than external.
So depending upon what you're doing, it is easy to make use of 1TB of space.
Now compare that to my little netbook with a 4GB SSD and a 16GB SD card. I cut all of the fat for that thing, but it serves a very different purpose.