Two MacBooks, 10.11.4, one quick TimeMachine backup and one very slow
In our home office, connected to a Gigabit ethernet network on the same switch, we have two MacBooks. One is a 2011 MBP with a 1 TB SSD (third party), and one is a 2015 MBA with a 512 GB SSD (Apple). A Linux server on the same network runs Netatalk 3 providing the network backup drives, one for each laptop. The two TImeMachine network drives are on the same physical server drive. Both laptops are running 10.11.4 and are configured pretty darn close to identically. Neither laptop has any trouble finding the network backup drives, and they start running backups without problem.
The old MBP has been a workhorse and running flawlessly for years and years, including fast and regular TimeMachine backups. Single-file restores are done with some regularity, and they have always worked (I think only once did we use a full system restore from scratch, and it worked).
The new MBA, on the other hand, has had endless issues starting and completing backups. Last night, for example, I decided to simply reformat the drive on the server in case there was a hardware error affecting the MBA (I ran Linux fsck to check the filesystem before wiping, and it found no errors), and started full backups on both laptops all over again. The MBP has about 600 GB for the first backup, and it completed the backup in about 5 hours.
The MBA, with about 350 GB to back up, was still running this morning, and had completed less than half its backup. From the console log, it showed it was backing up about 5 GB/hour. Not only that, but incremental backups have been taking many hours (more than overnight), making it impossible to ever complete them because it has to get packed up and taken to work at some point in the morning. I.e. backups are just plain broken on this MBA.
The MBA connects to the network with a third party (Anker) USB3 hub that has a Gigabit Ethernet port on it. I downloaded iperf for the Mac and the Linux server and found that throughput is right about 900 Mbps, so I expect full bandwidth is achieved through that connection.
Any ideas what might be going on here? Diagnostics between the two to see what might be such a radical difference? Have tried changing network taps, in case of wiring/switch issues, no change. Rebooting, no change.
We have two other Macs also running backups this way, under 10.11.4, and they have also been problem-free. So it's just this one MBA, the newest one of the bunch, having problems.
MacBook Air, OS X El Capitan (10.11.4)