Time Machine could not complete the backup.

I am having this problem with a brand new Western Digital My Book Duo 16TB drive, connected over USB 3 to a 2009 Mac Pro (Innatack USB 3 card). I also had a 2TB drive in an external USB 3 "toaster" dock.

One problem may have been that the WD RAID being on the same UPS as my Mac Pro was causing power issues. I got a warning about "UPS reports low power" which was weird because the UPS was plugged into the wall. Maybe during a 100-120 MB/sec backup operation, if insufficient power is available, the RAID drives go into an error state. The solution there was to plug in the RAID on a different power outlet.

However that wasn't the only thing.

I also found that my Sabrent USB 3.0 SATA Dual Bay Hard Drive Docking Station may have been causing issues. One drive being backed up was connected via the Sabrent also over USB 3. Perhaps the Innatek card is a piece of garbage or some bugs in OS X 10.8.5 (I know its old) just led to the inability of the USB host controller to handle both the reading AND writing. Moving the SATA drive from the Sabrent into a bay inside my Mac Pro solved that issue.

Now it's backing up like a champ.

See, getting "disk i/o errors" are not always a matter of what third party software you have installed. And it's not always a question of "your hard disk is failing". Sometimes it's literally "the drive is not getting enough power for peak operation" or "the host controller, she cannae handle the load!"

I have to say though... seeing "16TB available" was awesome, and while I know 100MB/sec might be slow by modern SSD standards, having my 6TB of photos and videos backup in less than 24 hours is a small miracle, LOL.

What kind of drive is it?

Run Repair Disk on the backup volume in Disk Utility, if you haven't already done so. It will take a long time, maybe hours.

G4Dualie (Michael) wrote:

. . .

I created this comment to add one more voice to the discussion regarding Time Machines inability to recognize whether a drive is read/writeable.

Time Machine may have set it to read-only because it found a problem.

Sounds like there's a problem with the drive, perhaps beginning to fail. 😟 If Disk Utility fixes it, but later on it turns up suspect.

You may see it in the system.log.

There are some similar threads in this forum.

Or, you could just try what two experienced users have suggested.

Perhaps ZFS is the answer to my problem?

Why didn't I think of that? Port ZFS to the Mac, then reverse-engineer Time Machine to work with it. Problem solved. Be sure to let us know when you're ready to release your code.

While we're waiting, others finding this discussion might want to peruse the Apple documentation that pertains to the issue:

If Time Machine is unable to complete a backup, make note of any alert message that appears. You can also open System Preferences and in Time Machine preferences click the red "i" icon for more information.

Messages you may see after clicking the red "i" include:

"The backup volume is read only."

Attempt to repair your Time Machine's backup:

1. Choose System Preferences from the Apple menu.
2. From the View menu, choose Time Machine.
3. Move the slider bar to "Off" to disable Time Machine backups.
4. Open Disk Utility located in/Applications/Utilities

1. Mount your external hard drive.
2. In Disk Utility, locate your external backup drive. Your backup drive should mount in the Disk Utility sidebar when attached to your Mac.
3. Select your external backup drive.
4. Click Repair.

Time Machine: Troubleshooting backup issues

The fact is that under some circumstances, diskarbitrationd(8) can update the 'rw' (read-write) mountflag of an HFS volume to 'ro' (read-only) if the filesystem fails an assertion. That's not Time Machine; it's the Disk Arbitration framework. Such failures occur because the device hosting the volume has thrown an error. If it happens once, the correct response is to repair the volume directory, assuming it needs repair and can be repaired. If it happens repeatedly, the correct response is to suspect a hardware fault and test with another drive.

G4Dualie (Michael) wrote:

. . .

TM isn't resetting my drive to read-only, you made that up.

No. I don't know (and didn't say) that's necessarily what's happened in this case, but that it does happen, and might well be what's causing the problem here. Your system.log may clarify what's going on.

But, I want help from the person a Level 7 goes to, Pondini. Nothing personal, nor am I challenging you or your knowledge but quite frankly your TM advice is fundamentally basic common Apple-sense.

Then you have some choices:

Call AppleCare. See the blue box in Reporting a Problem to Apple.

Use Apple's BugReporter. See the green box in Reporting a Problem to Apple.

Get a Developer's membership ($99/year), so you can download and test beta software, and file bug reports. That would let you test "point" upgrades to Lion, and/or Mountain Lion.

G4Dualie (Michael) wrote:

. . .

You said it MAY have happened, followed by your proclamation that perhaps the drive is about to fail and you based that on what I wrote! You didn't ask for logs, or press for further information, like you have done with evey other thread in this forum.

Correct. I recommended starting with Repair Disk, as Linc did, because that usually fixes it, and you'd answered that recommendation with the rather snippy "It's easier to unmount and mount the drive." If you'd made it clear you'd already done that, with a clean result, I would have gone further.

I'm done here.

As you wish. Be sure to post back when you solve it.

Yes, I know this is a very old thread, but I wanted to reply with my experience with the advice in the preceding question. I used Repair Disk, which didn't solve the problem. I then tried unmounting and remounting the drive, which did solve the problem. For a week or two - then the drive is again declared read-only until I unmount and remount it. While still in what TM claims is "read-only" mode, "Get Info" shows the disk read and write for me and for admin, but read-only for everyone; it will not allow me to unlock and change the permissions. This is under Mountain Lion - I don't recall it happening pre-Mountain Lion.

Mountain Lion seems in general to have done something weird to the disk directories, since after upgrading to ML, SuperDuper! was so completely fouled up about which disks were mounted and which weren't, and which disks it should be using, that I had to fully delete (using MacCleanse) and reinstall SD - now it works just fine. Hopefully that will continue, and not periodically get fouled again up like TM seems to do. Of course, I do realize that the SD problem may be totally unrelated to the TM problem (possibly SD plist or something got corrupted in the Mountain Lion update), but the symptoms have enough in common to be somewhat suspicious.