Mac Pro error codes

I/O Error sounds nebulous, but is in fact a very specific error. Your drive (Macintosh HD in this case) could not read valid data after as many as 1,000 re-tries.

The block in which those data reside has gone bad, and your drive may be junk, or could possibly be able to be fixed with a long erase of Write Zeroes. But you will not know whether the drive is re-usable until you have endured an all-afternoon long erase. And long term trends of large samples of drives suggest that the drive will continue to decline and become unusable within 6 months or less.

The best way to make rapid progress in to obtain a new drive, place it on a drive sled in your Mac Pro silver tower, and install Mac OS on it anew. Then migrate your files and Applications from your Backup, if possible. If you do not have a Trusted Backup, shame on you. This is your WakeUp call -- buy another large slow drive while you are up and enable Time Machine on it.

If you have a MacBook Pro, you will also need an external enclosure in which to mount the new drive while you do the install. If you skip the external enclosure part, you remove the tools needed (Recovery_HD) to do the new Install.

Once you have a new drive with Mac OS on it, you can salvage your files (if needed) at your leisure, and use the full power of MacOS and additional Applications such as Data Rescue to do so. Remember that over 350,000 of the files on the bad drive are MacOS files, and these are installed, unmodified, when you do a new Install -- MacOS does not modify itself. Preferences are stored in .plist files in your User-Id folders.