Short answer: the file is almost certainly a 32-bit float WAV, the standard in pro and Splice packs. The SP-404 MkII rejects it on SD-card import with no explanation. Here’s the full picture, and how to fix a whole pack at once.
1. 32-bit float WAV. By far the most common. Professional and Splice packs are usually rendered as 32-bit float. The SP-404 MkII only reads 16-bit integer audio on SD-card import, so a 32-bit file is rejected outright.
2. Unusual sample rates. Packs at 96 kHz or 88.2 kHz can fail. The device runs at 48 kHz internally; anything else needs resampling first.
3. FLAC and other containers. FLAC, M4A/AAC and similar compressed formats are not accepted by the direct SD-card import at all. (See converting FLAC to WAV for the SP-404.)
The SP-404 MkII has two ways in, with different rules. Direct SD-card import is strict, accepting only 16-bit linear WAV/AIFF/MP3; everything else comes back as “Unsupported File.” The official SP-404MK2 desktop app is broader, taking FLAC and M4A and down-converting on its own, but it’s built around transferring projects, not batch-prepping arbitrary loose packs.
The device runs everything at 16-bit / 48 kHz internally and conforms imports to it, so the cleanest approach is to convert to that target before the SD card ever sees the files.
| Codec | Linear PCM WAV |
| Bit depth | 16-bit (fixes 32-bit float, 24-bit) |
| Sample rate | 48 kHz (or 44.1 kHz) |
| Channels | Mono / stereo preserved |
By hand: open each file in a DAW (or use ffmpeg/sox in
the Terminal), convert to 16-bit linear WAV at 48 kHz, and re-export. Fine for a few files,
miserable across a whole pack.
The fast way: It’s 404, yo! batch-converts an entire pack in one drag, it tells you in plain language why each file would fail (“32-bit float → 16-bit”, “96 kHz resampled”, “FLAC → WAV”), converts only what needs it, and preserves your folder structure for the SD card. Native macOS, fully offline.
Related: Convert 32-bit float WAV for the SP-404 · FLAC to WAV for the SP-404