> In the case of this old PC hard drive, files of varying sizes were written to disk then later moved or deleted, leaving a "hole" of available space between other used regions. As the disk starts to fill up, you might try to create a new file that doesn’t quite fit in one of those smaller areas, and you’d be out of luck. You’d need to "defrag" in order to reclaim those open blocks which are too small to hold the new file when they are split up, but could be big enough when they are contiguous.

While that is pretty much what heap fragmentation is about, the failure mode of disk fragmentation is less drastic. The file system will just split the file contents across multiple smaller free spots, making it possible to use the whole disk no matter your write pattern. The issue is that now the file is no longer contiguous, so reading the entire file takes longer. Much longer if we are talking about old HDDs.