You could also try Select Columns when you open the file
Script created by it
Open("$SAMPLE_DATA/Big Class.jmp", Select Columns("name", "age", "height"));
This will let you limit the column selection, but I'm not sure if it will work for you.
Open also has other options to limit the data, you can open first x rows, last x rows or random x rows
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/CrimeData.jmp", First(10));
dt = Open("$SAMPLE_DATA/CrimeData.jmp", Random(10));
dt = Open("$SAMPLE_DATA/CrimeData.jmp", Last(10));
and instead of selecting specific columns, you can ignore specific columns
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp", ignore columns("age"));
One of the Open items in scripting index has some of these examples
-Jarmo