0  3338  3335   5  -5  0   148   92 vflush D+   co    0:00.09 umount /wabe 

I did the umount while doing a:
102  3325  3322   0  -5  0   220  296 physio DL   co    0:07.64 fdd -fill if=/dev/rst0 bs=126b 
102  3326  3322   1   2  0   420  132 netio  S    co    0:05.60 tar Bbtvf 126 - 

Additional umounts hang in the same way.  And of course since the disk is tied
up, the reboot fails.  df shows it is kinda a big file system...
    Filesystem  512-blocks     Used    Avail Capacity  Mounted on
    /dev/rccd0c   11768250 11179838        0   100%    

But this is how the umount left things:
Filesystem              512-blocks     Used    Avail Capacity  Mounted on
/dev/sd1a                    46190    25162    16408    61%    /
/dev/sd1g                   202046   177690     4150    98%    /usr
/dev/sd1h                  1503804   754618   749186    50%    /usr/src
				 0        0        0   100%    
With /sbin/mount showing:
/dev/sd1a on / type ffs (local)
/dev/sd1g on /usr type ffs (local)
/dev/sd1h on /usr/src type ffs (NFS exported, local)
 on  type