This article originally appeared in TidBITS on 2009-10-02 at 3:33 p.m.
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Solving the Mystery of Disk Utility's Failure to Erase USB Drives

by Glenn Fleishman

I was already tired, and then my generally lovely boys gave me a hard time bathing and getting to bed while my wife, Lynn, was out. Finally the kids passed out, and I moved on to backing up Lynn's hard drive with SuperDuper; the battery-charging circuitry was failing again in her MacBook despite an Apple fix, and she needed to take it into the Genius Bar again.

Of course, this is when Disk Utility told me it couldn't format an external Iomega 300 GB drive I had brought home from the office for the backup task.

Lynn's MacBook is still running Leopard, and despite trying many, many pieces of advice scattered across the Internet, Disk Utility continually told me, "Disk erase failed: input/output error." How useful. How user-friendly. Apple has no advice on the topic in its massive KnowledgeBase, either.

"Well," thought I, "let's try formatting with Snow Leopard's version of Disk Utility." It was infinitely helpful, providing a dialog that read: "Disk Erase failed with the error: POSIX reports: The operation couldn't be completed. Cannot allocate memory." Thank you so very much for such clear and comprehensible error messages, Apple.

After 45 minutes of Googling and grumbling, I finally realized that the Iomega enclosure had both FireWire 400 and USB 2 interfaces. I dug out a FireWire cable, plugged it in, and all was golden. I was able to erase and partition at will.

(An unrelated but widely discussed issue was that you might need to change the partition type from Apple Partition Map to GUID Partition Table in the Partition view's Options dialog in Disk Utility. GUID Partition Table is generally the right choice for Intel-based Macs.)

I later found some additional background advice on this topic from other users: it seems that there's some incompatibility introduced in Leopard that prevents Mac OS X from talking over certain USB drive interfaces.

Based on comments and discussions all over Mac sites dating back to the release of 10.5 Leopard, you have three choices:

I'm writing this article partly to attract Google searches - not for the hits (not at all!), but so that if you search for the error messages above, you'll find an article with some answers!

And yes, when my wife came home, she was quite grateful that I had conquered the dread formatting errors.

(See the comments on this article for more solutions that our readers have found for this and related problems.)