Usb drive is constantly active: why? I recently bought a large USB drive and stored about 300GB of new data on it.1 I haven't changed anything on the drive, and I'm not reading any files right now. But the drive is active -- I hear the read/write heads flicking back and forth like mad. What on earth is OS X doing?

I assume it has something to do with metadata. But it would be great to have a sense of what kind of metadata is being generated, and whether I can stop it (or would want to). I can see the amount of disk space used slowly creeping up!

1. I'm not sure if it's relevant, but the data consists of page scans and OCRed text from about 350,000 books from HathiTrust, stored mostly in zip files.
It's probably Spotlight indexing the drive, which is going to include full-text indices of those PDF scans if they've been OCR'd.

If you want detail--to see the status of the indexing, for example, there is a handy chart at http://commandlinemac.blogspot.com/2008/12/spotlight-on-command-line.html