Which Hard Disk? Selecting modern hard drives

A year ago, deciding what external storage to use with a Mac was relatively straightforward. Whatever your aspirations to large SSD RAID systems, they remained expensive and only worth the investment when you needed to handle very large files extremely quickly. For Time Machine backups and similar, it was a matter of selecting the best value hard drives.

Then along came High Sierra with APFS, which was going to support conventional rotating hard drives as soon as some performance issues were fixed (according to the APFS engineers at WWDC, June 2017).

Now we’re in limbo. As it stands and ships, APFS is still not supported on hard disks (or Fusion Drives), only on SSDs. Time Machine still won’t back up to APFS, and in its current form looks as if it never will. Apple hasn’t guaranteed that it will release a version of APFS which is supported on hard disks, has given no inkling of timescale, and hasn’t given any indication whether a future version of Time Machine will support APFS as its backup drive.

If you’re looking to buy or replace external storage for use as a Time Machine backup drive, you could easily find yourself with an obsolete system in just a year or two, particularly if macOS 10.14 brings Time Machine 2, which expects APFS for its backup, and APFS is never supported on hard disks. Although that may seem unlikely, six months ago it seemed absurd that Apple would ship APFS without support for its own Fusion Drives.

Assuming then that you’re looking for some conventional rotating hard disks to populate a RAID or work as external storage, what should you be looking for?

The best place to turn to for experience is Andy Klein’s latest report on the hard drives in use at Backblaze. They provide a cloud storage service which is hosted on almost 100,000 hard disks, and each quarter and annually Andy is kind enough to publish full details of their experience to date.

During 2017, Backblaze has been progressively replacing 2-4 TB drives with 8-12 TB units, although the majority remain 4 TB. They have been doing this as their experience with these larger drives has built, with growing confidence.

Backblaze buys drives in bulk, and for a number of reasons has limited its suppliers to Seagate, HGST (formerly Hitachi, now a part of WD), and WDC (Western Digital, as used to be). Over the nearly five years that Backblaze has been keeping figures, eight drive models have clocked up more than a million drive days in use:

  • Seagate ST4000DM000 4 TB, at over 35 million days,
  • HGST HMS5C4040BLE640 4 TB, at over 9 million,
  • HGST HMS5C4040ALE640 4 TB, at nearly 9 million,
  • Seagate ST8000DM002 8 TB, at over 4 million,
  • HGST HDS5C4040ALE630 4 TB, at over 4 million,
  • Seagate ST8000NM0055 8 TB, at over 2.5 million,
  • Seagate ST6000DX000 6 TB, at nearly 2 million,
  • WDC WD30EFRX 3 TB, at over 1 million.

The highest annualised failure rate from those drives was the sole WDC product, at 5%, followed by the drive used most, the Seagate ST4000DM000 4 TB at 3%. The other Seagate models came in just over 1%, and the HGST models at less than 1% and more typically around 0.5%. However, Blackblaze’s experience with HGST drives is almost confined to 4 TB. As failure rates are model-specific (indeed, batch-specific) rather than generic to a manufacturer, as they install large drives from HGST they may encounter higher failure rates.

If I were looking to buy hard disks now, I’d look first at HGST 4 TB models such as those listed above.

There are some other important points about Backblaze’s experience.

First, their drives spin continuously, they don’t keep getting spun up and down. Spinning up and down are the most wearing parts of a hard disk’s life. If you can keep yours spinning as long as possible, they’re likely to last well. This is particularly relevant to several of Apple’s desktop models such as iMacs, whose Energy Saver pane appears unable to put the system to sleep without putting hard drives to sleep too: if that’s the case with your Mac, disable system sleep, and let your Mac run continuously as much as possible.

Next, Backblaze has invested a great deal in cooling and conditioning their data centres, so that their drives don’t run hot. Hot drives tend to die young, so keeping your Mac and external drive housings comfortably cool and at a stable temperature is well worthwhile.

Finally, for smaller volume purchasers, when populating a RAID avoid installing drives from the same batch in it. Batches tend to fail at about the same time, so spreading your drives across different batches should save you the problem of two or more drives failing in a short space of time.

And let’s hope that whatever else macOS 10.14 brings, it continues to support backing up to external hard disks without any associated penalty.

Thanks to Andy Klein and Backblaze for sharing their invaluable experience and data.