This browser cannot open a USB stick

Reading a rekordbox volume needs the File System Access API (showDirectoryPicker), which Safari and Firefox do not implement and which has no polyfill that can reach a mounted drive. Chrome or Edge on a desktop will work. The engine still runs, and the file picker plays a single file — just without a grid, cues or playlists, which all live in the stick's database.

Continue anyway

fx off

Latency is what the browser reports (baseLatency + outputLatency), not an estimate. Underruns are blocks the callback could not fill; callback rate is quanta against wall clock and should sit at the sample rate. The stick is never bulk-copied — only export.pdb, then three files per track, dropped again once the decode has landed.