Using 9i on Win2K.

Had a corrupt UNDO datafile so I created a new one, made that the UNDO Tablespace for the database. Restarted the database without problem and the system is now up and running.

When I came to drop the old UNDO tablespace I got an error:

ORA-01548: active rollback segment '_SYSSMU1$' found, terminate dropping tablespace

What transactions could be active? No users are logged in?
I looked at V$ROLLNAME ...

SQL> SELECT * FROM v$rollname;

---------- ------------------------------
11 _SYSSMU11$
12 _SYSSMU12$
13 _SYSSMU13$
14 _SYSSMU14$
15 _SYSSMU15$
16 _SYSSMU16$
17 _SYSSMU17$
18 _SYSSMU18$
19 _SYSSMU19$
20 _SYSSMU20$

And this rollback segment doesn't even exist. The V$TABLESPACE view shows the TBS as ONLINE but when I try to offline it it says:

ERROR at line 1:
ORA-01191: file 2 is already offline - cannot do a normal offline
ORA-01110: data file 2: 'J:\ORACLE\ORA92\ADMSDEV1\DATA\AC_UNDO01DEV1.DBF'

If its offline, how can it have an active RBS?

Any ideas?