I recently bought an external hard drive (like 2 weeks ago)- I put two partitions on it, one set as a Time Machine backup (HFS+) and one as file storage (FAT32 to make compatible with Windows XP boot on my Macbook Pro). When I first got it, I remember being surprised by how quiet it was. I filled it up and now, whenever I plug the drive in, it makes a sound like it's constantly reading (reminds me of the sound old, old computers used to make when thinking a lot, i.e. with the blinking orange light). As soon as I start 'using' it (for instance, turning Time Machine backup on, etc.) it suddenly starts running quiet again. I made sure to make it 'invisible' to Spotlight so it doesn't index the drive... what else could be accessing the drive and making it work so hard? The constant 'grinding' of the drive is really bothersome!