I/O error -36
Posted by Graham Perrin on 10 December 2006
A friend’s Mac presents the following error when he tries to copy a file from one disk to another:
- Are both disks hard, and Mac OS Extended (HFS+)?
- Or, is one disc optical?
- In which direction is the copy failing?
Error -36 basically equates to an I/O error, but it should be interpreted in context.
- In some cases the error is symptomatic of bad media. If the error occurs whilst copying from one hard disk to another, then you probably do have bad media.
- In some cases, I/O is properly restricted. I vaguely recall seeing -36 a few weeks ago, whilst using Finder to backup protected DVDs.
Some protected content can be handled by Finder.
If the protection is great enough, then DVD drive hardware and/or firmware may simply say “YOU’RE NOT GETTING IT” to most applications.
Such denials/failures are sometimes non-explicit. Think of a dirty/scratched CD: there’s nothing on it to say that it’s dirty, it’s just troublesome to work with (the computer doesn’t “get it”). So, we have generic error -36 and it’s up to us to guess or figure out the explanation.
Apple’s Disk Utility can:
- verify and usually repair critical metadata (the directory of a disk)
- verify and repair permissions
- display S.M.A.R.T. status for some disks.
Whilst Disk Utility handles metadata it does not necessarily verify the integrity or readability of data (your files), nor does it check for bad blocks.
Amongst the original CDs/DVDs that came with your computer, or with an AppleCare product, you might have Micromat TechTool Deluxe. If you have this, or something like it, you can check for bad blocks. Depending on the size of the disk etc., the block checking routine can be time consuming.
DVD drives, decoders and raw access
Most DVD drives have an on-board decoder. When you use Apple DVD Player software:
- data passes through the drive’s decoder
- you’re forced to observe any region code that applies to the disc
- and so on.
Other software (VLC media player comes to mind) may interface with the drive at a different level —raw access— but modern drives with RPC-2 are making this increasingly difficult. If the drive allows it, VLC media player will read from the disc and use software (not the drive’s decoder) to
In the VideoLAN Wiki I found a suggestion that Linux users may modify the permissions of the block device that communicates with the DVD drive:
chmod 666 /dev/dvd
In Mac OS X: Finder logically prevents you from going to
ls -l /dev/disk*
lists all disks, including those that are not mounted. The number of the block device for the DVD drive will depend on how many other disks are present. To check the current number of an optical disk:
Disk Utility | select the disk | File menu | Get Info
Whilst it may be possible to modify the permissions of a device, you should not do so.
Copy protection in other contexts
Some people have problems with files that have been successfully ripped, which makes me wonder whether protection extends beyond DVD drives.
Apple certainly use the Trusted Platform Module (TPM) but I’m not aware of its use in this context.
References and further information
On other RPC2 drives that DO allow raw access, it might take VLC a long time to crack the key. So just pop the disc in your drive and try it out, while you get a coffee. RPC1 drives should ‘always’ work regardless of the regioncode.
Is your dvd drive one of those new RPC2 drives ? If so, you’re doomed. You can’t bypass the protection (unless you find a way to downgrade the firmware which might help, but sometimes voids the waranty)
AFAIK not all RPC-2 drives disable raw data access. IIRC this only became an issue with the most recent hardware revs, and earlier Macs still had RPC-2 firmware, but worked with VLC.
a ton of bad sectors on the disc as a form of copy-protection
That last one is curious. It includes discussion of bugs in Apple firmware for DVD drives, but concludes with a simple observation that the user may have been using dual-layer media that was not supported by the drive.
On 8 Jul 2002 I noted:
Retrospect has performed a backup (AppleShare over AppleTalk to single file backup set on our server CENTRIM-A) and is currently comparing the data. Retrospect log so far shows that System [suitcase] could not be read, “error -36 (i/o error, bad media?)“.
On 20 Dec 2004 I noted:
The Finder cannot complete the operation because some data in “April showers.cat” could not be read or written.
(Error code -36).