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You are here: MacNN Forums > Software - Troubleshooting and Discussion > macOS > I/O error during boot

I/O error during boot
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Join Date: Jan 2010
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Jan 28, 2010, 05:32 PM
Quick summary of facts:
*MacBook Pro (unibody)
*running 10.6.2 (upgraded from 10.5.7 which it came with)
*boots from system disks fine; boots into single-user mode fine
*can't get any further because of disk0s2 I/O errors
*no problems found by fsck or DiskUtility

Hello. My MacBook Pro (OS 10.6.2) is less than six months old; I've never had any major problems with it. Today it wouldn't awake from sleep, but this didn't alarm me unduly — it's been far too long (I admit) since my last reboot; I had far too many browser windows open (including some which I think had hung); and last night I had let power run down to 0%, so it had gone into safe sleep ("hibernation") mode. I was kicking myself for being an idiot/slob; but I didn't think it a big deal.

OK, but when I forced a manual restart, the computer wouldn't finish booting -- the little circle of lines just kept pulsing. I reset PRAM (no change); I booted into safe-user mode and ran fsck; no errors were found. But then I tried to continue the boot, and I kept seeing a series of disk0s2 I/O errors.

Since then I've run Apple Hardware Test (from the system disks) -- no problems found. I've run disk utility (from the system disks), and tried repairing both the disk and permissions; but no problems were found there either. When you've booted from the System CD, there's a menu option to choose the system software from which to boot the next time, and my hard drive shows up fine -- there's no indication that the computer can't 'see' the system software. But when I choose my hard drive and reboot from it, the disk0s2 I/O errors are still there.

So at this point I should probably think about reinstalling the OS from scratch. I could either do an 'archive and install' or I could back up my hard drive (using firewire target disk mode) and then do an 'erase an install.' But obviously both of these are last-resort sorts of things I'd rather put off until absolutely necessary. So my question is -- is this worth all the hassle of setting up an apple store appointment? (My local store is always too busy for walk-ins.) Is there any chance that they'll have any ideas? Or will I just spend large amounts of time/hassle to have them tell me what I already know?

Join Date: Jun 2000
Location: California
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Jan 28, 2010, 07:18 PM
I'm thinking you should back up the disk before doing anything further. I/O errors suggest a hardware issue. If you have a current Time Machine backup (to a separate disk) then you're covered.

Boot from install DVD -> Disk Utility. Select the drive icon (not the volume icon below it) and check the bottom of the Disk Utility window. What does the SMART Status say? Anything other than "verified" would be bad news.
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Jan 29, 2010, 05:01 PM
Just dealt with this earlier this week. I/O errors generally mean that the disk is failing. Even if the SMART status "verified". If you have a backup you are good to go. If not, you want to boot up from another disk, mount your internal drive, and retrieve as much info out of your Home directory as you can. ASAP. You then will need to get the internal drive replaced.

And yes I know. It sucks.

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Join Date: Apr 2005
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Jan 29, 2010, 07:31 PM
You are experiencing a failing hard drive, most probably a bad sector or two.
I had one bad sector on my 3rd party 500GB drive in my MBP, it was in such a place that it would cause the whole system to beachball while it tried to read the data. I left it for hours before it could complete a totally up to date time machine backup.
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