Oracle EM detect a disk full, but the system has plenty of free space

My enterprise manager instance on Oracle 10.2.0.4 throws the following alert on a regular basis:

Name=db4
Type=Host
Host=db4
Metric=Disk Utilization (%)
Disk Device=sd1
Timestamp=Oct 4, 2011 2:15:50 PM GMT
Severity=Critical
Message=Disk Utilization for sd1 is 96.12%
Rule Name=Host Availability and Critical States
Rule Owner=SYSMAN

The DB is hosted on Solaris 10. The datafiles are stored on /home and the binaries on /opt. And there is free space on both volumes...

-bash-3.00$ df -h
Filesystem size used avail capacity Mounted on
/dev/dsk/c0t0d0s0 9.9G 4.3G 5.4G 45% /
/devices 0K 0K 0K 0% /devices
ctfs 0K 0K 0K 0% /system/contract
proc 0K 0K 0K 0% /proc
mnttab 0K 0K 0K 0% /etc/mnttab
swap 38G 968K 38G 1% /etc/svc/volatile
objfs 0K 0K 0K 0% /system/object
sharefs 0K 0K 0K 0% /etc/dfs/sharetab
/usr/lib/libc/libc_hwcap1.so.1
9.9G 4.3G 5.4G 45% /lib/libc.so.1
fd 0K 0K 0K 0% /dev/fd
/dev/dsk/c0t0d0s3 20G 355M 19G 2% /var
swap 38G 152K 38G 1% /tmp
swap 38G 28K 38G 1% /var/run
/dev/dsk/c0t0d0s4 20G 13G 6.8G 66% /opt
/dev/dsk/c0t1d0s0 825G 611G 205G 75% /home

Is this alert from EM a false positive ? A bug that I should patch ? The really weird thing is that I have another database running on the same server, with EM installed, and that secondary EM doesn't throw the error.