I had this "-50" problem when copying executable files from a mac to an ibay. It is a permissions issue on the hidden files created by the macintosh for its resource forks.
If you go to the ibay configuration and change the "Public access via web or anonymous ftp" to "none" the problem goes away.
It goes away because the mac mounts the files subdirectory of the ibay (for which it has write permission) instead of ibay directory (for which it only has read permission).
When using macs you should use separate ibays for web
content and file sharing. This should be in the e-smith manual.
Chris