Guide - Two exactly the same Ultra star HDDs

If you own two Ultrastar hard drives that are exactly the same in model number, capacity, and even firmware revision—but notice a difference in their speeds—it can be puzzling. How can two "identical" enterprise-grade drives behave so differently during read/write operations, benchmarks, or real-world file transfers?

Here are some key reasons why two Ultrastar HDDs, even with matching labels, may show different performance levels, along with steps to diagnose and resolve the issue.

1. Background Processes and Workload History

Hard drives often run background tasks, such as error correction, reallocation of bad sectors, or internal diagnostics. If one drive has recently encountered issues or has been heavily used, it might be slower due to ongoing background operations.

Check the SMART status of both drives using tools like:

CrystalDiskInfo (Windows)

smartctl (Linux/macOS)

Look for attributes like:

Reallocated Sectors Count

Current Pending Sector

UDMA CRC Error Count

A higher number in one drive could indicate it's working harder in the background.

2. Different Power or SATA Cables/Ports

Even if the drives are identical, a bad or lower-quality SATA cable, or connecting the drives to different SATA ports (3Gb/s vs. 6Gb/s), can affect performance.

Verify:

Both drives are connected using high-quality SATA 3 (6Gbps) cables.

They are plugged into the same type of SATA ports on your motherboard.

No port is set to operate in a slower compatibility mode (e.g., IDE or SATA I).

3. Partition Alignment and File System Overhead

Drives formatted differently or with non-aligned partitions can experience reduced performance. For example, if one drive was cloned from an older system using 512-byte alignment while the other was freshly formatted with modern 4K alignment, performance can vary.

Use tools like:

MiniTool Partition Wizard

diskpart (Windows) or gdisk (Linux) to check alignment

Compare file system types (NTFS, exFAT, EXT4) and fragmentation

4. Different Firmware Optimization or Modes

Some Ultrastar models have multiple OEM versions or firmware options, especially if they were supplied to different vendors (e.g., Dell vs. HP vs. retail versions). Even with the same model number, performance can differ slightly due to:

Head parking settings

Power-saving modes

Read-ahead buffer optimization

You can use tools like hdparm (Linux) or vendor-specific utilities to inspect or adjust these parameters.

5. Temperature and Cooling

HDDs tend to throttle or slow down when overheating. Use a utility like HWMonitor or smartctl to compare temperature readings. If one drive is consistently hotter (due to airflow, enclosure design, or position), that could explain the slower speed.

Final Thoughts

Even if two Ultrastar HDDs look identical on the surface, their performance can differ due to multiple subtle variables—background wear, cabling, firmware settings, or physical conditions. To isolate the issue:

Swap cables and ports

Test in the same PC environment

Benchmark using the same tool (CrystalDiskMark, HD Tune, etc.)

Monitor SMART data and temperature

If the difference persists, and especially if one drive shows errors or abnormal SMART values, it may be a sign of early degradation. Let me know the specific model (e.g., DC HC550, HC560, etc.), and I can guide you more precisely.