For example, if you're starting lots of programs at once they'll need to read lots of files scattered around your drive.įrom the drive's perspective, it's seeing commands like 'read block #56, read block #5463, read block #14, read block #5' Blocks This doesn't have to mean truly random it really means 'not sequential'. Random access means there's no obvious pattern to the reads or writes.

If you're downloading a file, it gets downloaded and written to disk from beginning to end.įrom the disk's perspective, it's seeing commands like 'read block #1, read block #2, read block #3, read byte block #4' 1. For example, if you're watching a video, you would load the video from beginning to end. Sequential access means you read or write the file more or less one byte after another. There are, broadly, two different ways you might access a file. In CrystalDiskMark, 'Seq' means sequential access while '4K' means random access (in chunks of 4kB at a time, because single bytes would be far too slow and unrealistic 1).

What you are running into is typical of mechanical HDDs, and one of the major benefits of SSDs: HDDs have terrible random access performance.

Crystal disk mark ssd test