Why unused files files get fragmented? (Windows 7)

Question

Every now and then i defrag my HDD partitions to maintain my PC, and each time i notice fragmented files that i haven't read/wrote-to in a while. For example npm-cache, i haven't used Node for ages!

I have Windows Search disabled, and i can't think of any action or application that would randomly read random files on my HDD, all that uses my HDD on Idle are uncharted system processes, constant and small read/writes.

What causes unused files to get fragmented?

Answer 1

Windows7 by default schedules an automatic defragmentation on a regular basis (daily iirc). Defragmentation will move files adjacent to fragmented files it wants to rewrite, and will often leave you with more fragments than you started with, but hopefully in less used files. This becomes more true as the disk fills up. Multiple passes can eventually reduce the number of fragged files. You can never predict what files will be touched or moved around during a defragmentation.

Answer 2

One possible candidate is that Windows Update can can affect files that the user isn't touching.

The Windows update process can touch or check for the existence of files in order tell if an available update should be downloaded. The application of the update can touch files that you may not expect due to some dependency within the update.