The manual is very clear that stereo and multitrack is supported to a USB hard drive via the USB A connector on the top surface.
It is also clear that hard drives are supported but USB keys (aka thumbdrives) are not.
Not quite:
"Scene, Library and Show memories - All Qu mixer memories are
compatible. Files created on one model can be transferred via USB key and
loaded on another..."
And then:
"USB Audio - A USB hard drive formatted on one model can be used on the
other. Stereo and multitrack Qu-Drive recordings are compatible. USB B
audio streaming is compatible...."
When I see "USB key" I immediately think thumbdrive....not USB hard drive.