I’m not sure about the largest size. But it has been successfully tested with USB drives of 32GB.

Review the device compatibility list (under "other downloads") and pick the largest tested size.

https://usa.yamaha.com/products/music_production/synthesizers/montage/downloads.html

I can confirm through repeated and thorough testing using a Yamaha Montage 8 (runningV2) that it indeed can and does with ease.. 100% format,save to, access and load files perfectly with absolutely full compatibility and no errors using:

SAMSUNG 128GB USB 3.0 Flash Drive Fit (adheres with USB 2.0) | Model Code# MUF-128BB/AM

4X the storage capacity of the SAMSUNG 32GB models tested in the past.

This is so cool..it provides direct and immediate onboard access to literally a massive amount of sound libraries collections, sampled sounds, and FM converted sounds ready to go with room to spare!

It's awesome to have all of that with me at all times with quick access. Just thought I'd pass along the info. Enjoy!!

That said, even if a 32GB drive can only use 29GB due to filesystem overhead, this is still 16x more memory than Montage can fit fully loaded. So 32GB seems like a reasonable place that Yamaha stopped their testing although I understand that some would want to use the USB stick for more data than would load into Montage (such as unedited/scrubbed samples which can get big -- but really is just a temporary store before you trim them down to fit within the memory limits and still leave room for other libraries to be loaded in other slots).

Thanks for passing along the test data.