I'm guessing this is the same GTK2 app that's copying data that several monks discussed with you in the CB? If so, let me see if I can remember some more background to fill in the readers of this thread.

You have one HD from which your copying, and you're copying to up to 49 others. The target drives are on seven seven-port USB hubs plugged into a single seven-port USB hub which is plugged into a single USB port on the PC-class machine. You're wanting to speed up the copying, but for some reason you're not going to be using IO::AIO as was suggested. You can't use dedicated drive duplication equipment because your software is supposed to be part of a package product with the machines and USB hubs.

If that's about right, that background information may help the monks come up with useful suggestions sooner rather than spending a bunch of time figuring out your specific situation. It's always good to let the people helping you know if you're dealing with particular unusual circumstances and constraints so that they don't waste their time and can help you and other Seekers of Perl Wisdom better with less burnout.