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Solving the Mystery of Disk Utility's Failure to Erase USB Drives

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:

  • Switch to FireWire. That only works, of course, if your drive has a FireWire interface.
  • Boot off a Mac OS X 10.4 Tiger installation disc, and use Disk Utility to reformat the drive while booted in Tiger. That's a clever, if slightly tedious, approach, although it won't work for machines too new to allow a Tiger boot.
  • Remove the drive and put it into another enclosure, preferably one with a FireWire interface.

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.)


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Comments about Solving the Mystery of Disk Utility's Failure to Erase USB Drives
(Comments are closed.)

John Nemo  2009-10-02 17:38
Thanks for your good introduction to this essential topic, Glenn. I avoid USB-only drives for myself and my tutorial clients for this and another reason: FireWire is much faster. The popular MyBook drives are the worst offenders on both counts, from my experience. It is so easy and affordable to purchase and use combo FW800/400/USB drives, there is no reason for intelligent Macintoshers to do otherwise. Macsales.com, WiebeTech.com, Verbatim.com, G-Technology.com, and other companies have fine products that I use daily. Keep this conversation going. USB-only drives are bad choices. [Nemo / MyMac.com Reviews Editor]
Isn't it going to be hard to find a firewire port soon. I thought apple was getting away from them and moving to only USB.
Glenn Fleishman  2009-10-02 18:46
There's no announced plan. Some new models removed FireWire; some continued to have them.
Adam Engst  An apple icon for a TidBITS Staffer 2009-10-02 18:59
And to expand on what Glenn said, the MacBook model that removed FireWire was discontinued and replaced with a MacBook Pro that got FireWire back. So that's a good sign for the continued survival of FireWire.
Frumius  2009-10-03 03:26
And, just in case it wasn't understood, the remaining MacBook also has a FireWire 400 port. In fact, every model except the MacBook Air has either a FW400 or FW800 port.
Stephen  2009-10-02 19:38
I had a similar issue with a friend's usb hard drive on her Leopard machine. It would not reformat as one partition. I divided it into two partitions and they formatted perfectly.
Simon Klein  2009-10-04 09:28
I too was able to reinitialize a pc drive as mac by partitioning it. I bought a Cavalry drive (admittedly, it is slow). I tried different strategies for two weeks to no avail. Then I discovered a technical bulletin on the manufacturers website "Repartitioning and reformatting from FAT32 to Mac OS X Extended" . The method is simple and works
bill hamlin  2009-10-02 21:52
I got the same "Can't allocate memory" statement. It was just as useful to me as to you. I did change the jumpers on the drive from cable select to Master and then all went well. Of course, you have to take things apart and put back together, but it worked. I then backed up the drive with no problem.
Got the suggestion from a neighbor guru type. Don't know why worked, but did.
Khürt  2009-10-03 06:26
I had a similar problem with a G-Drive Q. Switching to USB resolved the issue but the FireWire never worked. What is odd is that I have another G-Drive Q with the exact same specs that works fine on FireWire.
I have an external enclosure with 4 PATA drives in it. In 10.5 I was only able to mount these drives via the FW400 port, and even then sometimes they would not mount if I had my iSight plugged in at boot. After installing 10.6 I was able to mount the drives via FW800 and there's no conflict with the iSight.
Dave Bursik  2009-10-04 00:04
I ran into a similar problem with an external Iomega 2.5" drive w/USB & FW400. It works fine on my MacBook Pro w/USB, but only works with FW400 on my 15-in aluminum G4 PowerBook. I attributed this to a hardware issue since the drive is powered by the port (although it has a jack for an optional power supply). Testing with an external power supply might determine if it's a hardware problem (i.e., if it starts working over USB with the PowerBook).
Ken Elliott  2009-10-04 14:27
The problem is that most USB drives come formatted for Windows (Master Boot Record).
In Disk Utility instead of trying to Erase, select Partition, then change the Volume Scheme to 1 Partition, in Options select GUID Partition Table, now click Apply to format the disk for Mac.
Glenn Fleishman  2009-10-04 15:14
That only works in some cases. This wasn't one of them. The drive was already formatted for Mac, and I tried for both the Apple and GUID partition tables.

The posts on other forums indicate that changing partition types sometimes helps, sometimes doesn't. Since booting into Tiger always helps, it's clearly a Leopard bug carried through in a different way into Snow Leopard.

It's important to look at lots of situations before making a blanket recommendation. Hence this article.
Bob Sanderson  2009-10-05 22:38
A family member bought a really cheap USB-only 500GB external hard drive upon which my wife tried to copy 35GB of photos. We needed to reformat the drive before we could copy the files. However, it showed up as "Read Only", and we couldn't erase it in Disk Utility.

By accident I ejected the drive by clicking on the eject icon in the Finder sidebar. The drive disappeared in the Finder, and the icon disappeared from the desktop.

However, Disk Utility still showed the drive as connected, and it was now able to erase and reformat the drive. We were then able to copy the data onto it. The new format was FAT-32, but I don't think that matters.

Maybe this workaround will work for you.
B. Jefferson Le Blanc  2009-10-06 04:47
I find that my portable external drives don't always mount when (I think) they should. In the case of USB drives this may be due to insufficient power on the USB bus, particularly with laptop computers. For this reason I always keep an appropriate power adaptor handy for the drives - using external power usually does the trick. Sometimes a computer restart is required as well. I haven't erased any of these drives recently so I couldn't say if the power issue applies as well in this case. But it's worth considering when you're running through alternatives.
JerryNaples  2009-10-06 11:51
My experience is that I've solve this problem by Using Partition in Disk Utility I would have also picked the Guid Partition Map
R Brown  2009-10-07 18:11
I have found that using Partition works as well. Recently with a stubborn drive, I create a single partition set to "free space" first. When that was done, I repeated the single partition process using Mac OS Extended (Journaled) with GUID Option set. Drive is working well now. Copied over 20 GB of files as a test.
Ran into this problem today.
Bought a USB2 Seagate 1TB drive formatted for Windows NT - it showed up on the desktop fine (MacBook Pro/Leopard). So I decided to use Disk Utility to change the formatting to MAC OS extended journalled and it immediately wigged out and the drive would no longer mount. Disk Warrior could do nothing.
I decided to use my recent purchase of iPartition and it had no problemreformatting the drive and changing the partition map to Apple.
So it seems to me that, (here at least), Leopard is fine with USB drives - perhaps it is a problem specific to Disk Utility?
Martin  2009-10-12 07:59
I have an old Formac drive with an FW400 interface where I was getting the POSIX error with Snow Leopard. After reading through the earlier comments I tried 'erasing' it on my wife's old laptop running Tiger. Worked perfectly - now mounts on SL without demur. It's only a workaround though, so I hope someone will come up with a proper Snow Leopard solution soon.
I just ran across this error. One option I found that worked for me was using the 'pdisk' command in the terminal. The steps are as follows:

1. Open Terminal
2. Type "diskutil list", that will show you the attached drives. Find the offending drive (mine was /dev/disk2)
3. Then type "pdisk /dev/disk2". Replacing whatever you got in step 2 for "/dev/disk2"
4. Then type "i", and take the default options.
5. Type "w" to write the partition map
6. Type "p" to see the partitions, one should be very large, and labled Apple_free. Mine was #2
7. Type "c" to create a partition. For first block, type 2p (or whatever number you got from step 6). For length, type the number listed under length in step 6, then type a name (If you want to make multiple partitions, enter a smaller number for the length and then repeat this step for each partition, using 3p, 4p, etc as your start)
8. Type "w" to write it, then type "q"
9. You'll get an error, but then you should be able to use Disk Util to fix it.
Stuart Moore  2009-10-27 03:36
I had a similar problem - I couldn't persuade my disk to reformat using either firewire or USB, or using any of the other suggestions above.

I rebooted, and then managed to reformat first time - in my case, firewire, 2 partitions (1 FAT) with MBR partition table.