After reading the manpages for ntpd(8), ntpd.conf(5), & nmeaattach(8),
I thought I had enough information to use a USB GPS device as a time
source.

In /etc/ntpd.conf, the only line left enabled is:

        sensor cuaU0

In /etc/rc.conf.local, the only lines are:

        nmeaattach cuaU0
        ntpd_flags=""

Yet upon reboot, I see the following message displayed during initialization:

        nmeaattach: open: /dev/cuaU0: Device busy

I can observe the NMEA sentence traffic through cu(1), so it is
unclear what I have missed.  Any insight would be appreciated.

Thanks.

Jim