I can run it on my system, coincidentally we have an F: drive on our network. I can append without issue. If it compiles the problem is in the string itself and how it is intrepreted at runtime. Are you setting it in code yourself ( lcFile = "XX" ) or reading it in from another file or parameter etc... You could have hidden CHR(?) in it. Try and cleaning the string up by searching for control characters before executing the command. ALLTRIM it, run a few AT() or OCCURS() and see if anything is in it. I have read in strings from .INI ( basically a text fiel ) file via an API calls and they append string terminators ( which causes Foix to choke ) because they are written in C and you can't see them.

Other than that I am out of ideas.