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What are "Input/output" errors, and how can I resolve them? |
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When copying to or from damaged media, or a disk with a damaged filesystem, CCC may report "Input/output" errors. When these errors are encountered, CCC will present an alert indicating:
"CCC detected "Input/output" errors during the clone."
Or, as of version 3.3.5:
"CCC has detected a physical problem with one of your volumes."
Correspondingly, the CCC.log file will usually list the files or folders that CCC was attempting to read from or write to when the error occurred. To view CCC's log file, choose "CCC log" from CCC's Window menu. Here are some example input/output error messages that you might see in the CCC.log file:
rsync: mkstemp "/Volumes/Backup/Applications/iChat.app/Contents/Resources/Italian.lproj/BuddyInfo.nib/.designable.nib.MtvjHR" failed: Input/output error (5) rsync: rename "/Volumes/Backup/Applications/iChat.app/Contents/.Info.plist.uHwdgM" -> "Applications/iChat.app/Contents/Info.plist": Input/output error (5) rsync: recv_generator: mkdir "/Volumes/Backup/System/Library/Caches" failed: Input/output error (5) rsync: recv_generator: failed to stat "/Volumes/Backup/System/Library/Colors/Apple.clr": Input/output error (5) rsync: read errors mapping "/Users/mike/Movies/1984.mov": Input/output error (5) ERROR: Users/mike/Movies/1984.mov failed verification -- update discarded.
The exact errors may vary, and some may be duplicated, but they always contain "Input/output error".
Read errors caused by damaged media
Read errors are typically a result of media damage — some of the "sectors" on the hard drive have failed and Mac OS X can no longer read data from them. Read errors can occur on the source or target volume. When read errors occur, the file or files that are using the bad sector must be deleted. If the read errors occurred on the source volume, you should restore the affected files from a previous backup, reinstall the affected application, etc. Once you have deleted the files that were called out in the CCC.log file, you should be able to run your backup task again with success. If a directory was affected by the damaged media, you should replace the affected directory from a good backup then run a filesystem repair utility (e.g. Disk Utility's "Repair volume" tool) to clean up any related filesystem damage.
Disk Utility does not perform a scan for media errors
While it is generally a good practice to run Disk Utility's "Repair volume" utility when you run into problems with your disk, note that Disk Utility does not scan for bad sectors, it only checks the health of the filesystem. Additionally, the SMART status that is reported in Disk Utility will report "Verified" unless the drive is in a pre-fail condition — i.e. failure is imminent. Bad sectors will not be reported by Disk Utility.
Individual sector failures are not uncommon, and not necessarily indicative of imminent drive failure. A full-volume backup of your hard drive is a great method for detecting media problems with sectors that are in use because it requires reading data from each of those sectors.
Errors on read or write caused by physical drive malfunction
Occasionally input/output errors are reported when a mechanical failure is preventing Mac OS X from reading a portion of the hard drive. Failures such as these are sometimes characterized by unusual noises coming from the hard drive (especially repetitive clicking noises) and slow performance of the affected hard drive. CCC may also appear to "hang" when it encounters these kinds of errors. If you are booted from the affected hard drive, you may notice that Mac OS X seems quite unresponsive.
If your source or target hard drive is experiencing physical malfunction, you may have a narrow window of opportunity to back up the data from that disk to another hard drive. Time is precious; components could fail at any moment rendering the drive completely unmountable. Read activity is stressful on a dying volume, especially a full-volume backup. We recommend that you immediately back up the files that are most important to you. When you have backed up the most important data, next try to do a full-volume backup. When all of your important files are backed up, you may also try running a filesystem repair utility (e.g. Disk Utility's "Repair volume" tool) if signs of imminent failure are not present (e.g. particulary noisy drive, or loud, repetitive clicks). In any case, though, we recommend that you replace the affected hard drive.
What if the dying drive's volume won't mount?
More often than not, you're completely out of luck. I provided tech support at a University many years ago and had the opportunity to witness many failed drives. Occasionally we were able to revive a hard drive for small amounts of time by letting the drive cool down (somewhere cool and dry, not cold) and then powering it up attached to a service workstation (e.g. don't attempt to boot from it, you may not have enough time). When a drive doesn't mount, it typically goes to the recycle center or to DriveSavers if the data is worth the recovery cost. If you're reading this paragraph now because you're in this situation, my heart goes out to you.