Manatee

How to repair a corrupted MP4 that won't play on a Mac

· 6 min read

An MP4 that refuses to play on a Mac has one of two problems: the file is structurally damaged (usually a missing or broken index from an interrupted recording, download, or copy), or it is perfectly intact but uses a codec QuickTime Player cannot decode. The first needs a repair; the second needs a conversion or a different player. This post shows how to tell them apart in about a minute, then how to repair the file on your Mac without uploading it anywhere.

Match the symptom to the cause

The way the file fails tells you a lot.

  • QuickTime says the file is damaged or could not be opened, and every other player fails too. Structural damage. The container is broken. Go to the repair section.
  • QuickTime says the file isn’t compatible, but another player plays it. A codec problem, not corruption. Convert it or keep using the other player. See QuickTime says the file isn’t compatible.
  • The file opens, shows a duration of 0:00 or something absurd, and the play button does nothing. Broken index. Repairable if the data is there.
  • Audio plays, picture is black. Usually a video codec the player cannot decode, occasionally a damaged video stream. See How to fix a video that plays audio but shows a black screen.
  • It plays and then freezes or stops at the same point every time. Localized damage in the middle of the file. Repairable, with a gap.
  • The file is 0 bytes. Nothing to repair. The data was never written or the copy never started.

How an MP4 gets corrupted

An MP4 is a set of boxes. The big one, mdat, holds the compressed audio and video. A second one, moov, is the index: sample sizes, offsets, timestamps, codec parameters. Most recorders write mdat as they go and write moov only when recording stops, because they cannot know the sizes until then.

That ordering is the root of most corruption. Stop the process before moov is written and you get a file that contains every frame and no way to find them. Typical triggers:

  1. A camera, drone, dashcam, or phone that lost power or filled its card while recording.
  2. A screen or webcam recording ended by a crash.
  3. A download that was interrupted. Browsers sometimes leave the partial file with the final name.
  4. A copy from a card or external drive that was ejected early, or a cloud sync that was interrupted.
  5. Physical problems with the drive after the file was written.

Some recorders write moov at the front (a “fast start” layout) so the file streams well. Those files survive truncation better: the index exists, it just points past the end of the file, and a player will often play up to the cut.

Quick checks before you repair

  1. Look at the size. Command-I in Finder. A ten-minute 1080p phone recording is typically several hundred megabytes. If the file is a few kilobytes, there is no video in it.
  2. Go back to the source. If the file came from a phone, card, or cloud folder, check whether the original is still there and intact. A fresh copy beats any repair.
  3. Copy it to the internal drive. Work on the copy. Do not run anything against the only copy on a memory card.
  4. Try one other player. This single step separates corruption from codec trouble. If any player plays it normally, skip repair and convert instead.
  5. Optional, for Terminal users: ffmpeg -i broken.mp4 -c copy remuxed.mp4 rewrites the container without re-encoding. It fixes files with minor box-level errors and does nothing for a missing moov, because ffmpeg cannot read the input without it.

Repair the MP4 with Manatee

A real repair reads the raw stream in mdat, identifies the frame boundaries from the codec’s own markers, reconstructs timing, and writes a new moov. That is what the Repair mode in Manatee does.

  1. Open Manatee. You can use every feature free for the first 24 hours, with no card and no email, which is enough to fix the file and see the result.
  2. Drop the MP4 into the window, drag it onto the menu bar icon, or right-click it in Finder and choose Fix with Manatee.
  3. Let the job run. Progress shows in the job list. Large files take longer because the whole stream is scanned.
  4. Check the outcome: Fixed, Partial (with a concrete figure, for example the first eleven minutes of a nineteen-minute recording), or Not fixable.
  5. Open the repaired copy, which sits alongside the original. The original is untouched; Manatee never modifies it.

Nothing leaves your Mac. The repair engine is bundled in the download, and the only network request the app ever makes is a license check.

Reading a Partial result honestly

Partial means the file had a clean run of decodable frames and then stopped making sense. The common shapes:

  • Truncated recording. You get everything up to the cut, minus the last incomplete frame.
  • Damaged region in the middle. You get the video with a gap, or in some cases the section before the damage and the section after, depending on how the decoder resyncs.
  • Video and audio of different lengths. Streams are written in interleaved chunks, and the cut rarely falls on a chunk boundary for both. The shorter one sets the playable length.

The missing material was never in the file. No tool, and no amount of re-running the repair, will produce it. Read Partial recovery explained if you want the details.

When to stop

Do not keep trying if:

  • The file is 0 bytes or implausibly short for its claimed length. Look for the original elsewhere instead.
  • The file was deleted and you are working from a recovered fragment. Undelete tools sometimes return the right size with the wrong contents, and there is nothing a repair can do with that.
  • The MP4 is protected by DRM, which no repair tool will or should touch.
  • The drive it lives on is making noises or throwing I/O errors. Back up everything first; repair later.

Questions

Does repairing an MP4 reduce its quality? No. The frames are copied, not re-encoded. The repaired file has the same resolution, bitrate, and codec as the original. Quality only drops if you later choose to convert or shrink it.

Why does the MP4 play on my phone but not on my Mac? That is almost always codec, not corruption. Phones record in HEVC by default and Macs handle HEVC, but some combinations (10-bit, certain HDR profiles, or third-party players) fail. See Why some videos play on iPhone but not on Mac.

Can Manatee recover a deleted MP4? No. Repair works on a file you already have. Recovering a deleted file from a card or disk is a different job for a different kind of tool, and it has to happen before the space is reused.

How long does a repair take? It depends on file size more than duration. A short phone clip finishes in seconds. A long 4K recording of several gigabytes can take a few minutes because every byte is scanned for frame markers.