Time Machine failing with (null) error I'm getting the following after about half an hour of Preparing Backup:


Setting security information: Operation not permitted
Error processing extended attributes: Input/output error Error moving - -- disk image from /var/folders/zz/zyxvpxvq6csfxvn_n0000000000000/T/com.apple.backupd.WpUkVn/iMac.tmp.sparsebundle to /Volumes/TimeMachine/iMac.sparsebun... 
Failed to
create disk image /Volumes/TimeMachine/iMac.sparsebundle, status: 512
Backup failed with error 20: 20 
Network destination already mounted at: /Volumes/TimeMachine 
statfs call failed, error: 2 No such file or
directory


I had Time Machine working but now nothing. A few extra things:


I have completely formatted the partition. 
I have tried deleting the Time Machine preference file. 
I am using a router running Netatalk. I am aware that this is not a supported backup solution.


Thanks guys!

iMac (Retina 5K, 27-inch, Late 2014), OS X El Capitan (10.11)
I also use netatalk/afpd, backing to a USB-mounted drive.  If I partition the drive on OSX first (hfsplus, no journaling), then mount it under Linux, chmod &amp; chgrp it to the TimeMachine userID, and share with netatalk, then I get the same error you report (after "preparing backup", error is "Setting security information: Operation not permitted"), and backup fails every time.  

However, if I stop netatalk, repartition the USB drive to a single EXT4 partition, mkfs.ext4, remount and chmod, chgrp, and share with netatalk, then I do not see this error and TimeMachine backups work fine.

I expect that the problem is userID/permission related, but since ETX4 is working for me, I have not pursued it.