Two exactly the same Ultrastar HDDs - different speeds

If you've installed two Ultrastar hard drives of the same model, size, and specifications but are noticing different speeds, you're not alone. Many users report similar issues, especially when working with enterprise-grade drives like Western Digital Ultrastar. But how can two seemingly identical HDDs perform differently? Let’s explore the most common reasons.

1. Manufacturing Variations

Even if two HDDs share the same model number, they may come from different manufacturing batches. Tiny differences in firmware revisions, component suppliers (e.g., platters or controllers), or even factory locations can impact performance slightly.

Example: One drive might have firmware v7.01, while the other has v7.02.

Result: Slight differences in how caching, power management, or thermal throttling are handled.

2. Pre-Use Wear or Power-On Hours

If one of your Ultrastar drives was used before, even briefly (e.g., in a server or test bench), it may already have some background wear or sector reallocation. This can slow performance compared to a brand-new unit.

Use tools like CrystalDiskInfo or smartctl to check SMART data and power-on hours.

Drives with reallocated sectors or high load/unload cycles can show reduced performance.

3. System Bottlenecks or Configuration

Sometimes, it’s not the drive’s fault but the environment it's in:

Is one connected via a different SATA port or using a different cable?

Is your system running both drives in different power profiles or performance modes?

Are they being tested or used under the same conditions (e.g., idle, background tasks, OS caching)?

4. Temperature and Thermal Throttling

Ultrastar drives often include thermal sensors that engage throttling when temperatures rise to protect the drive.

If one drive is located in a hotter zone of the case (e.g., near GPU or PSU), it may throttle more aggressively.

Use temperature monitoring tools to compare idle and load temps.

5. Testing Methodology

Different benchmarking tools (CrystalDiskMark vs. ATTO vs. real-world file transfers) measure different aspects of performance—sequential vs. random, read vs. write, cache effects, etc.

Ensure both drives are tested under identical, isolated conditions.

Final Thoughts

Two “identical” Ultrastar drives may vary slightly in speed due to firmware, usage history, or environmental factors. If both pass SMART tests and perform within acceptable thresholds, minor speed differences are usually nothing to worry about. For mission-critical use, always keep backups and monitor drive health regularly.