different speeds - Two exactly the same Ultrastar HDDs

It can be puzzling and even concerning when you have two exactly the same Ultrastar hard drives—same model number, capacity, and firmware—yet they perform at noticeably different speeds. Whether you're noticing longer load times, inconsistent benchmark results, or slower data transfers from one of the drives, there are several technical factors that could explain this discrepancy.

Let’s explore the most common reasons why two seemingly identical Western Digital Ultrastar HDDs might operate at different speeds.

1. Different Manufacturing Batches or Revisions

Even if the drives have the same model number, they may have been manufactured at different times or factories. Small hardware changes or microcode updates can affect performance. WD, like other drive manufacturers, often refines manufacturing processes over time. These updates aren't always reflected in the visible model number but may impact seek times, cache algorithms, or power management behavior.

Check the manufacturing date and location on the drive label. A unit made in 2022 might use slightly different components compared to one produced in 2024, even under the same part number.

2. Variations in Firmware or Microcode

Ultrastar drives often receive firmware updates for bug fixes, performance improvements, or compatibility changes. If one drive is running older firmware, it could explain performance differences.

To check and update firmware:

Use Western Digital’s Dashboard utility or third-party tools like smartctl.

Verify if both drives are running the same firmware version.

Updating the slower drive (or matching versions) can sometimes resolve inconsistencies.

3. Health and SMART Attributes

Even new or lightly-used drives can exhibit performance differences if one is experiencing minor health issues, like weak sectors or higher error correction overhead.

Run a full SMART diagnostic on both drives. Pay attention to:

Reallocated sector count

Seek error rate

Interface CRC errors

Read/write error rates

Even if the drive hasn’t failed, early warning signs can cause reduced throughput or hesitation during reads and writes.

4. Power Supply and Cable Quality

Believe it or not, inconsistent power delivery or poor SATA cable quality can lead to one drive underperforming. If the slower drive is connected via a lower-quality cable or shares a power rail with other power-hungry devices, it might throttle performance to maintain stability.

Try swapping cables, SATA ports, and power connectors to see if the slower performance follows the drive or remains tied to the system configuration.