The usb stick on t29 had been offline for a couple days, apparently because of a usb disconnect like was happening on t38 before it was replaced. (ISFS-609) So I rebooted it and now the usb stick is working again.
The t0b and t0t usb sticks had reached more than 50% full. I removed the hotfilm data from t0t (that had already been copied off), and I changed the rsync removal window to keep 21 days of data on the sticks instead of 60.
nagios is all green again.