Lazaclip

FX3A RSV file: recovering a take that was cut off

Updated 7 November 2026

The FX3A stopped in the middle of a take: the battery or the external power gave out, the camera was switched off by accident, or a card came out too early. On the card there is now a file ending in .RSV instead of the .MP4 you expected, and your editor won't open it. This guide explains what happened and how to get the take back, step by step.

If you shoot on the original FX3, the steps are the same; there is a separate FX3 guide as well.

What happened to the file

While the FX3A records, it writes the picture and sound into a working file with the .RSV extension. When you press stop, the camera adds the index (the table that says where every frame and every piece of audio sits in the file) and renames the file to .MP4. If the power goes before that, the index is never written. The recording itself is in the file, but players and editors can't find their way through it. More background: What is an RSV file?

The FX3A records XAVC S, XAVC HS and XAVC S-I into .MP4 files, so an interrupted take in any of these formats ends up as an .RSV.

Before anything else: protect the card

  • Don't format the card, in the camera or on a computer, even if the camera suggests it.
  • Don't record the next take on that card. New recordings can land on the space the interrupted take occupies.
  • Don't delete the .RSV, however strange the file looks.
  • Copy everything to a computer first. Put the card in a card reader and copy the whole PRIVATE folder. The video files are in PRIVATE/M4ROOT/CLIP; a copy means you can try things without risking the only original.

Check the second slot

The FX3A has two card slots that take CFexpress Type A or SD cards. If it was set to record to both cards at the same time and only one card was pulled out, the other card may hold a finished .MP4 of the same take, so check it before you repair anything. If the battery died, both cards normally hold an .RSV; copy both, and you have two copies of the same data.

If the second slot was only used for overflow, the take is on the card that was active at the moment of the cut.

The free first try: the camera itself

Some Sony cameras try to recover an interrupted clip on their own when you put the card back into the same camera with a charged battery. We can't tell in advance whether your FX3A will do it for your take, and it doesn't always happen, but it costs nothing. After you have copied the card:

  1. Charge the battery, or connect reliable power.
  2. Put the card back into the same FX3A, in the same slot, and switch it on.
  3. If a message about recovering or repairing data appears, let it finish and don't switch off.
  4. Check PRIVATE/M4ROOT/CLIP on a computer again for a new .MP4.

Rebuild the take as an MP4 on Windows

If the .RSV is still there, it needs a new index. Lazaclip is our program for exactly this (we make it, so this is our own product being described here). It runs on Windows 10/11, works offline and doesn't upload your footage:

  1. Open the card or the folder you copied. The program lists every .RSV and says whether it can be rebuilt.
  2. Click Preview 10 s to see the first ten seconds of your take, watermarked, in your video player. Scanning and the preview are free.
  3. With a license, save the full take as an .MP4 on your PC, or rebuild every .RSV on the card in one go.

Nothing is re-encoded: the program keeps the recorded video and audio as they are and only writes the missing index around them. The original .RSV is never changed.

Has it been tested with the FX3A?

No, not yet. Our tests so far use A7S III recordings (XAVC S, XAVC S-I and XAVC HS, rebuilt without a reference clip, with frames and audio identical to the original up to the last unfinished group of frames) and the original a7S. The FX3A uses the same XAVC S / XAVC HS recording format as those cameras. We haven't had FX3A files in our tests yet; the free preview shows whether the program can read your file before you pay.

What you get back

  • The take up to the last second or so before the cut. That final moment was never completely written and can't be rebuilt.
  • The sound as the camera recorded it, from the built-in microphone or the inputs you used.
  • An .MP4 roughly the size of the .RSV: save it to a drive with enough free space, not back onto the card.
  • S-Log3 footage still looks flat until you grade it, exactly like your other clips. XAVC HS needs an editor with H.265 support.

Avoiding it on the next job

  • Long event takes drain batteries faster than you expect. Change batteries between takes, not when the warning appears, or run the camera from external power that can't be knocked loose.
  • Recording to both slots at once costs a second card but gives you a second copy of every take.
  • Wait until the camera has stopped writing before you switch it off or open the card door.
  • Keep a spare card in the bag, so a card with an .RSV on it can be put aside untouched.

Shooting with an FX30 as a second camera? See the FX30 guide.