Code:
serout PORTB.7, T9600, ["OPR volume.txt", 13] 'open file
serout PORTB.7, T9600, ["RD", 13]  'read file
Not sure if RDF will always work in your case because the amount of characters are not constant. But try the above...
I missed the W earlier and I think once the file is opened the name is not used until after it is closed and you need to open it again.