On the JUNO-DS61, JUNO-DS76, and JUNO-DS88, your performance cannot be recorded directly to a USB flash drive in real time.
If you need to save your performance, record it into the Pattern Sequencer first, then export it to a USB flash drive (32 GB or smaller, formatted on the JUNO-DS) using the "SMF EXPORT" function. The exported SMF file is saved in the "EXPORT" folder of the USB flash drive.