Dramatic difference between hard disk transferrate "sequential" vs. "random" access?


New Member
I have connected my computer through USB 3.0 cable with an external (backup) hard disk (Hitachi HDS724040ALE640, SATA III, 7200rpm, 4TB, NTFS formatted, empty).

The USB controller in external case box and the
USB port on mainboard are USB 3.0 compatible.

When I benchmark now the transfer rate with well know, wide spread tools like
CrystalDiskMark (see: Download)
then I can see a huge difference between the sequential read/write and the random access read/write transfer rate.

See the attached snapshot (top 2 lines: sequential read/write; Bottom 2 lines for random read/write).

Is this a general observation?

Since users need on 99% of the usage a random access the sequential values are almost meaningless.
720-760 KBps (NOT: MBps) for random read speed? Is this normal?

USB 3.0 and SATA III shouldn't be the bottleneck.

Is there any way to improve the random access read/write speed in Windows?
E.g enabling Write cache or USB property value or driver setting?
Does defragmentation help (significantly)?

What has the most impact on the transfer speed with random access?


  • CrystalDiskMark 8.0.0 x64 [Admin].png
    CrystalDiskMark 8.0.0 x64 [Admin].png
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Last edited:


Microsoft MVP
Staff member
Microsoft MVP
The whole range of components to disk access in various modes (sequential and the many types of random) is very involved but one component alone (seek time - the time taken to identify the location of data and then move the heads to the required ) is typically many times hat of the actual transfer time (read/write). Random access requires far more "seeks" (typically one or more for each disk access) so therefore is significantly slower.