Hard drives and UREs (or “What’s really going on?”)

Matt Simmons, the Standalone Sysadmin comments:

Yesterday, I was reading a thread that asked about failure scenarios involving RAID-5, and like all discussions related to RAID-5, it was full of misinformation stemming from misunderstanding.

A lot of the comments seemed to think that because Unrecoverable Read Errors (UREs) were presented as probabilities across bits read, that having previously read a certain number of bits increased your chances of encountering a URE while rebuilding a drive. I meant to reply to explain what a URE is, and give an example of something that would cause a URE, and ended up writing a novel.

Read more at: Hard drives and UREs (or “What’s really going on?”)

About the author

Stephen Foskett

Stephen Foskett is an active participant in the world of enterprise information technology, currently focusing on enterprise storage, server virtualization, networking, and cloud computing. He organizes the popular Tech Field Day event series for Gestalt IT and runs Foskett Services. A long-time voice in the storage industry, Stephen has authored numerous articles for industry publications, and is a popular presenter at industry events. He can be found online at,, and on Twitter at @SFoskett.

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