You basically are testing the reading and writing of files from FTP sites. If you cannot read or write the files, you need to find out why.
You may also want to test timings (performance) by setting thresholds for acceptable file transfer rates. If the file transfer time exceeds one of those thresholds that you set up, you need to be aware of that as you might have a nasty bottleneck. Also from the performance side, you will want to look at the failure rate for connections and find out (1.) why that happens and (2.) if there is a pattern to the connection failures.
You will also want to make sure that unauthorized people cannot get into your FTP files, particularly if you do not allow anonymous access. (This is part of your general security testing.)