Updating Across A Partitioned View

This post concerns updating across a partitioned view, and not unlike others about this subject I am getting this error:

Msg 4436, Level 16, State 12, Line 1
UNION ALL view 'dbII.dbo.MyTable' is not updatable because a partitioning column was not found.

I am aware of the rules for defining a partitioning column, but interpreting them may have beaten me. So perhaps I haven't abided by all the rules. How to spot which one(s) from the view and table definitions? I suspect the CHECK constraint does not allow the ASCII function, but I can't see how to avoid using it given SYSCODE entries in one table are like "[A-Z]%" and in the other are like "[0-9]%".

Otherwise, I suspect it is because one of the tables has, by legacy, a text column and the view is casting it to varchar(MAX). I also suspect it is because there's a second column with a unique index. These aren't mentioned in the rules (are they?).

Here's the view definition:

SELECT SYSCODE, COL2, CAST(COMMENTS AS varchar(MAX)) AS COMMENTS
FROM dbo.MYTABLE
UNION ALL
SELECT SYSCODE, COL2, COMMENTS
FROM OTHERDATABASE.dbo.MYTABLE AS MYTABLE_1

And here are the table definitions:

-- Table in the database where view is defined
CREATE TABLE [dbo].[MYTABLE](
[SYSCODE] [char](12) NOT NULL,

[Code]....