Why do some of my disks get errors during certification?

If some of the disks you are certifying are getting errors, there are two possible explanations:

You have an unreliable batch of disks. Some manufacturers ship new hard drive technology before it is truly reliable. We have seen this occasionally with early shipments of very high-capacity drives, as well as with refurbished disks. If you are seeing failures across multiple new drives from the same batch, this is the most likely cause.

Your Mac has a hardware problem. In rare cases, we have seen users where more than 50% of their disks fail certification. On a Mac Pro — the only current Mac that supports additional RAM — this can sometimes be traced to bad or mismatched RAM. On any Mac, other hardware faults can also cause widespread certification failures.

You can test whether your Mac is the cause by moving your disks to a different Mac and re-certifying them. If they all certify successfully on the second Mac, the most likely explanation is that the original Mac has a hardware problem. You can investigate further by running Apple Diagnostics (hold D at startup) to check for hardware faults.

A note on certification time: Certification is thorough but time-consuming — plan on approximately one day per 2TB of capacity for hard disk drives. A 4TB drive can take up to 48 hours; larger drives take proportionally longer. SSDs and NVMe drives are much faster, typically completing a 3-pass certify in 4–8 hours for a 2TB drive, depending on interface speed.

Want to skip certification entirely? OWC pre-configured storage solutions — such as the ThunderBay and Gemini series — come with drives that have already been certified and tested before shipping. This means your enclosure is ready to set up and use right out of the box, with no certification wait. Learn more at owc.com.