"Operation must be an updateable query" error

folks,

i am getting the error:

 "Operation must be an updateable query"

when i try to run the following quey from within MS Access 2003.

UPDATE STUDENTS SET SSN = (SELECT SOONER_ID.Sooner_ID  FROM SOONER_ID  WHERE SOONER_ID.SSN = STUDENTS.Student_ID)
WHERE Exists (SELECT SOONER_ID.Sooner_ID  FROM SOONER_ID  WHERE SOONER_ID.SSN = STUDENTS.Student_ID);

here is the table structure

STUDENTS
------------
SSN
Student_ID

SOONER_ID
-------------
SSN
Sooner_ID


If i modify the query like so:

 UPDATE STUDENTS SET SSN = '01234567891'
WHERE Exists (SELECT SOONER_ID.Sooner_ID  FROM SOONER_ID  WHERE SOONER_ID.SSN = STUDENTS.Student_ID);

it works fine. i've checked and i do have read/write access to the .mdb file in question.

what is the problem here?

Thanks.