Never mind. I deleted everything and started over again from scratch today, and it all works as expected. Very mysterious!:unsure:

I have found that either of the following methods work. The second is MUCH faster:

1.

UPDATE ISERIES.Serial#.Library.File

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

2.

UPDATE OPENQUERY(ISERIES, 'SELECT * FROM...