zfs directory contents lost

Hi all,

I have created

	zfs create dbase/FrontBaseDatabases
	zfs create dbase_new/FrontBaseDatabases

	zfs set mountpoint=/opt/FrontBase/Database dbase/FrontBaseDatabases
	zfs set mountpoint=/opt/FrontBase/DatabasesNew dbase_new/FrontBaseDatabases

and filled both with lots of data. I then did

	zfs unmount  dbase/FrontBaseDatabases
	zfs unmount  dbase_new/FrontBaseDatabases

and hoped I could simply do

	zfs set mountpoint=/opt/FrontBase/Database dbase/FrontBaseDatabases
	zfs set mountpoint=/opt/FrontBase/DatabasesNew dbase_new/FrontBaseDatabases

again to remount the filesystems. But this does not work. If I do

	ls /opt/FrontBase/Databas
	ls /opt/FrontBase/DatabaseNew

I get nothing anymore (directories empty)! :-( Why? Are my files lost? How do
I get my files back? The "df -h" output indicates 70GB used so the files must
be somewhere, but where?

	df -h

dbase_new              134G    21K    64G     1%    /dbase_new
dbase                  134G    21K    64G     1%    /dbase

This is pretty urgent (has to run in a couple of hours again)! Please help!!
What have I done wrong?

Thanks a lot in advance!

Regards,

 Andreas