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Seagate Iron Wolf Pro 20TB Hard Drive

Seagate Iron Wolf 20TB hard drive

re: Installing an 18TB Hard Drive into the OWC Mercury Elite Pro
re: Formatting a new Hard Drive or SSD in Apple Disk Utility
re: Western Digital 18TB Ultrastar DC HC550 Hard Drive

Questions on storage, RAID, backup, reliability? Consult with MPG Lloyd.

This page evaluates the Seagate Iron Wolf Pro 20TB hard drive (ST20000NE000) for speed across the entire volume capacity as well as speeds for 32K to 512MB transfers.

The OWC Thunderbay 8 enclosure was used to house the drive. Its Thunderbolt 3 interface offers 8 bays with 4 SATA channels, which supports full performance simultaneously across all 8 drive bays. As such, it could house 8 X 20TB = 160TB.

CLICK TO VIEW: Recommended Storage at B&H Photo


Choosing a drive capacity

Hard drives slow down as they fill up. A good way to evaluate hard drive is by its average speed across the capacity that will be used, as well as how it performs at around 80% capacity usage, plus its consistency across the capacity.

When purchasing hard drives, MPG recommends capacities that will keep actual storage used at no more than 80% of capacity, so as to maintain reasonably high speed. For example, anticipation of up to 16TB of storage needs suggests a 20TB hard drive (16TB is 80% of 20TB).


Enhance your business' massive storage capabilities with the 20TB IronWolf Pro 7200 rpm SATAIII Internal NAS HDD from Seagate. Designed to be used with network attached storage systems up to 24-bays, the IronWolf Pro gives you massive capacity suitable for enterprise and commercial businesses. Sporting a 3.5" form factor, 7200 rpm rotational speed, and 256MB cache, you can experience quick data transfer rates essential for fast-paced environments. The IronWolf Pro also has an MTFB rating of 1.2 million hours, providing long-lasting lifespan, making this an ideal storage solution for your business.

AgileArray — Experience advanced power management with AgileArray which enables dual plane balancing and RAID optimization in multi-bay environments

IronWolf Health Management —  Actively protect your NAS with IronWolf Health Management that focuses on prevention, intervention, and drive recovery

Rescue Data Recovery — Included 3-year Rescue Data Recovery plan, you gain extra peace of mind for any mechanical, accidental, or natural disaster with 90% success rate of in-house recovery

Integrated RV Sensors — Maintain high performance within multi-drive NAS enclosures with integrated rotational vibration sensors that ensures proper performance suitable for your business needs

Multi-User Environments —  No matter the size of your business, the IronWolf Pro produces a workload rate of 300TB/year suitable for data uploading and downloading with confidence

24x7 Performance — The IronWolf Pro is designed to be accessed at any time whether you access your NAS remotely or at the jobsite

  • 20TB Storage Capacity
  • 3.5" Form Factor
  • SATA III 6 GB/s Interface
  • 256MB Cache 7200 RPM
  • 1.2 Million Hours MTBF
  • Up to 24-Drive Bay Supported

About hard drive performance

Hard drives slow down as they fill up. The outer tracks deliver more data per rotation than more inward inner tracks due to constant data density, but a much greater circumference in the outer tracks—more data per revolution of the platters. Thus performance drops steadily as the slower and slower inner tracks are used. This is true of ALL hard drives, but not SSDs.

Use the fill-volume command of diglloydTools DiskTester to determine performance across drive capacity for hard drives or SSDs, single drives or RAID of any kind.

TIP #1: to sustain higher performance for same capacity utilization, buy larger hard drives.

Creating a volume using less than the full capacity bounds that volume’s storage area to the faster tracks (first volume created gets the outer and fastest tracks, next volume gets the next-fastest tracks, etc).

TIP #2: to guarantee minimum performance for a fixed-size capacity, the first partition/volume will be the fastest. Use less than the full capacity to guarantee a bounded minimum speed e.g., a 12TB partition on a 16TB total capacity. The extra space can be ignored, or used for a 2nd much slower volume.

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Test results: Speed across 20TB capacity

Test mule was the 2019 iMac 5K with the OWC Thunderbay 8 enclosure.

The fill-volume command of diglloydTools Disktester was used, with 128MB transfer size.

For over 13 years, DiskTester has been the best means available to ascertain how a drive performs across its capacity. When working with RAID, graphing each hard drive individually can reveal performance irregularities that can cause hiccups in RAID performance when multiple drives are used—problem drives can be swapped out for more consistent ones. But here, only a single drive was available for testing.

  • On the fastest part of the volume, speed is very fast for a hard drive, around 275 MB/sec.
  • At 50% capacity speed is down to about 230 MB/sec—still very fast for a hard drive.
  • At 80%, about 175MB/sec—superb, since that is ~16TB of capacity usage.
  • Past 80% capacity, speed falls off to ~123 MB/sec at the very limits of the capacity.
  • At the 18TB mark, substantially faster than 18TB hard drives (~150MB/sec vs ~127MB/sec).


The Western Digital 18TB Ultrastar is nominally faster over the capacity by about ~7MB/sec. But that has no significance to real usage, and the Seagate 20TB has 2TB more capacity. Moreover, at the 18TB mark, the Seagate 20TB is still doing ~150MB/sec, about 18% faster than the Western Digital 18TB near its capacity.

Average Write speed:  218 MB/sec
Average Read speed:   220 MB/sec

Vertical scale is MB/sec. Horizontal scale is percentage of volume capacity eg 50% of 20TB volume is 10TB.

Seagate Iron Wolf 20TB hard drive: speed across 20TB volume capacity

Vs Western Digital 18TB Ultrastar

Superimposed below is the Western Digital 18TB Ultrastar.

The Seagate Iron Wolf Pro 20TB begins to have a significant speed advantage at around the 10TB mark, meaning that once there is 10TB on the drive, the 'Wolf' will eat the Western Digital drive in performance—see the widening performance gap. Indeed, the advantage is already emerging at around the 8TB mark (400 on the graph).

That gap would be much larger if 20TB in favor of the 20TB, were compared to a 16TB or 14TB hard drive.

This is a strong argument for going with a larger drive, but it applies to any pairing of larger versus small capacity drive. Meaning that a Western Digital Ultrastar 20TB drive would likely be as fast or faster than the Seagate 20TB.

Seagate Iron Wolf 20TB hard drive: speed across 20TB volume capacity, vs Western Digital Ultrastar 18TB

Test results: speed vs transfer size

Test mule was the 2019 iMac 5K with the OWC Thunderbay 8 enclosure.

The run-sequential-suite command of diglloydTools Disktester was used for this data.
disktester run-sequential-suite

Transfer performance for 32K and 64K transfers is as good as some (slower) SSDs. What is remarkable is just how fast the Seagate 20TB is for all transfer sizes. Even the dip at 64K transfers is still a commendably fast speed. There is some outstanding onboard drive caching going on here!

Seagate Iron Wolf 20TB hard drive: speed across volume capacity


Questions on storage, RAID, backup, reliability? Consult with MPG Lloyd.

The Seagate Iron Wolf Pro 20TB hard drive is an outstanding choice for ultra high capacity storage as a single drive. Users with greater capacity needs and/or fault tolerance needs and/or higher performance can use RAID-0 striping or RAID-1 mirror or RAID-1+0 stripe+mirror in an enclosure like the OWC Thunderbay 8 or OWC Thunderbay 4.

While Seagate markets the drive as a NAS drive, ironically most NAS setups will severely throttle the drive performance, since gigabit ethernet is only good for 125 MB/sec at best—slower than the drive when filled to capacity. A NAS setup should be done with 10-gigabit ethernet, even for a single drive setup.

Just four of these Seagate 20TB drives equates to 80TB in an OWC Thunderbay 4, which for those of us who started out at 5MB (5 megabyte!) hard drives, it is amazing to have right on your desktop!

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