And that’s all I know, really. I can download small files from the iTunes Store, but larger files (e.g. a 98 MB movie) fail with the message that the disk I am downloading to is full (err: -34). The problem of course is that it isn’t full at all: 18 GB free on the primary drive, 60 GB free on the external drive where I keep all my music.
A little Googling led me to this support thread which suggests restarting the Airport Extreme Base Station, or copying a dummy large file then deleting it, as potential workarounds. We’ll see.