FYI, we just used a 128GB USB thumb stick to do a SAVSYS to the USB drive.
IBM only tested up to 32GB but we mostly have 64GB thumb drives and also use SSD drives connected to a USB cable for backups. So we tested a 128GB USB thumb drive.
Slow as heck, but it worked!
Next time we'll try one of our 512GB SSD drive connected to a USB cable to see if that works.