UNIX for Dummies Questions & Answers

I'm new to coding on Unix but have coded with COBOL on mainframe for a number of years. This is driving me nuts. My select clause is like so:
SELECT GRPFILE ASSIGN TO EXTERNAL DISK WS-GRPFILE
ACCESS IS SEQUENTIAL
STATUS IS WS-SORTGRP-STATUS.
my WS declaration:
01 WS-FILENAMES.
05 WS-GRPFILE PIC X(26)
VALUE "/mpux/MCS/DATA/GRPFILE.SRT".

I can open the file with file status "00" and write to it with file status "00" until I have written 1024 records - at which point I get a file status 23 ?? (which used to mean to me that record key not found).
When looking at the physical file I see that nothing has been written so I coded a close when I wrote 1023 records:
IF CNTR = 1023
CLOSE GRPFILE
PERFORM EOJ
END-IF.
I got a status 42 - file already closed. I know for sure its already open but I inserted the following OPEN line giving:
IF CNTR = 1023
OPEN OUTPUT GRPFILE
CLOSE GRPFILE
PERFORM EOJ
END-IF.

Now I get a status 41! Its already opened and its already closed! I tried to "touch" the file so that it was already generated before the original open statement - still opens with a "00" but still gives the same end result. I believe this all has something to do with logical vs physical files but dont know where to take it. Anyone??

It would be helpful to provide

(a) the operating system you are using and
(b) the version of COBOL.

Code:

VALUE "/mpux/MCS/DATA/GRPFILE.SRT".

UNIX filesystems are normally case sensitive, does this match the file system?

yes - the file name and location is correct including caps, the host system is
HP Itanium B.11.23