I love the fact that my Airport Extreme can serve as a "poor-man's NAS" by making USB drives available as Samba shares. But when I wanted to back up one drive to another, I ran into terrible problems. Here's how I solved these problems.
I started out with two FAT-formatted drives. FAT is lame, but I've got Windows laptops and a MacBook Pro at the house, so I figured that if I ever needed to directly connect the drives to one of these laptops, FAT would be the most accessible. My plan was to use one drive as a primary drive and periodically back up to the second drive. (Hello, software RAID support for the Airport Extreme, Apple?)
I quickly ran into problems with one of the Windows laptops. It could connect to the drive on the AE, but the connection was not reliable.
I eventually traced this down to an incompatibility between the wireless chipset on this laptop (a Toshiba Satellite R15 tablet PC) and the AE. It's an Intel Pro/Wireless 2200BG chipset. Some folks have pointed to the Power-Save Polling as the culprit, but I was never able to get the built-in wireless to play nice with the AE.
So I bought a cheapo USB wireless adapter (Rosewill RNX-G1W) for the Toshiba, and things worked like a charm from that point on.
Once I got the primary drive working reliably, I needed to figure out the backup solution. I originally thought I'd just connect both drives to the AE, mount them both from a client laptop, and do a recursive copy between them. So naive.
I could not get this to work -- the laptop would always end up losing one or both volumes. Sometimes this would happen a couple of hours into the backup process.
I tried this from the MacBook Pro and from a Windows laptop. No luck. Under Windows, I tried using the Explorer, I tried Cygwin, and I even tried a special batch copy client for Windows. All failed.
So my next approach was to try connecting directly to a laptop and perform the copy. I tried it with OS X -- the drives eventually unmounted during the copy. Windows also had problems.
In desperation, I reformatted the drives as Mac OS Extended, thinking that maybe the MacBook Pro would handle its native file system format better. Still, I got the disconnects.
Finally, I thought of another approach: using the old backup standby of tar.
With the primary drive mounted on /Volumes/Primary and the secondary on /Volumes/Secondary, here are the commands I used:
mkdir /Volumes/Secondary/backup-2010-01-15
cd "/Volumes/Primary"
tar cf - * | ( cd /Volumes/Secondary/backup-2010-01-15; tar xfp -)
Eureka! It worked. No unmounting. It took a few hours to backup about 100GB. But it worked.
Gaining confidence, I thought I'd try it over the network with the drives connected to the Airport Extreme. But this failed with errors like
Device not configured
and
Socket is not connected
So it seems the way to get a backup is to disconnect the drives from the AE, connect them directly to an OS X machine, and use the tar commands to recursively copy the filesystem.
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