Why Does My iPhone File Transfer Stop Halfway?

Sometimes the transfer might fail because the file size is too large, or you're trying to move too many files at once. I'd recommend trying to transfer just 10 or 20 files at a time. Also, avoid multitasking—don't click around on your computer while the transfer is happening. Another tip is to move the files to your hard drive first and then to the USB; it could make a difference.

What if you try transferring your files to your computer's hard drive first? Then, once they're there, you can move them to the USB drive. This way, you can see if the issue lies with the direct transfer to the USB.

It sounds like you might be dealing with the PTP/MTP emulation that iPhones use, which can be pretty unreliable. Have you considered using Apple's official app for file transfers? It might help stabilize the connection and ensure the files make it over without interruptions.

Check if the USB stick is overheating, especially if it's a smaller one. Many of them can't handle continuous data writing for long periods. If possible, you could use a larger drive or even point a fan at the USB stick while transferring to keep it cool.