I found a good and fast solution for updating linked AS/400

It is the same solution that Ken LaRoy suggest

UPDATE OPENQUERY(ISERIES, 'SELECT * FROM Library.File WHERE field2 = ''00248000'' ')

SET field1 = RTRIM(field1) + ' Changed'  -- WHERE field2 = '00248000'

The last WHERE clause is unnecessary since it is included in the OPENQUERY SELECT

Very fast, even though the AS/400-table has million of rows.

/PG