Hi Karl,
First of all, when a USB is formatted not all of the memory will be available for use. I don't have a 64 GB device, but my 32 GB ones have 29.8 GB available for storage after formatting. You should be able to see the total space available on your device if you examine it on a computer (BTW, Yamaha keyboards will not be work with devices formatted as NTFS)!
However I don't think that the error you are getting is due to the total space available on the USB!
There is a note about the Data Backup function on page 105 of the Genos Owner's Manual stating:
• If the total size of the target data for
backup exceeds 3.9 GB (excluding
Audio files), the backup function is
not available. If this happens, back up
the User data by copying the items
individually.The backup function produces a single file (.bup), and I suspect that your data produces a file bigger than the 3.9 GB maximum.
Although you could copy individual items to get around this limitation, quite frankly I think that it is totally ridiculous to limit the backup file to what amounts to less than 10% of the total User drive memory space! I realise that by default the backup file does not include any audio files on the drive (although you can include them if you wish), but even so, it seems a bit silly to me!
Personally I prefer to back up my data as individual files, because if you have a problem with one or more files at some point, you can then restore just what is needed rather than having to restore everything from a '.bup' file.
However if Yamaha see fit to include a backup/restore function, they should make sure that it is fit for purpose, which in the case of Genos, it would seem is not the case! Maybe there will be a fix for this in the June firmware update.
Regards
Ian