File transfer interrupted: how to salvage a half-copied video

A half-copied video is a truncated file: everything up to the moment the transfer stopped is there, byte for byte, and nothing after it. If the source still exists, copy it again and ignore the damaged one. If the source is gone, the first part can usually be rebuilt into a playable video, as long as the file’s index was not the missing part, and a repair tool will tell you in seconds which case you have.

First, go back to the source

Before doing anything to the broken file, check whether a complete copy still exists. It is the only way to get the whole video back, and it is surprisingly often still there:

1. Phone or camera. AirDrop, a USB cable, or an SD card copy leaves the original on the device untouched. Look in Photos on the phone or on the card.
2. The sender. Messages, email, and chat apps keep the attachment on the sender’s side. Ask for it again.
3. Cloud. A stalled download from iCloud Drive or another sync service usually leaves the full file on the server; reopen the folder and let it finish, or download from the web interface.
4. Download managers and browsers. A stopped browser download can often be resumed from the downloads list rather than restarted.
5. A second attempt. A copy that failed because a drive was unplugged or a network share dropped usually succeeds when repeated with a stable connection.

If any of those gives you the whole file, delete the half-copied one and move on. The rest of this post is for when the source is truly gone: the card was reformatted, the phone was wiped, the sender no longer has it.

Confirm it is truncated and not something else

Select the file and press Command-I. Compare the size with what it should be (from the source, the sender, or a rough estimate: 1080p phone video is on the order of 60 to 130 MB per minute, 4K several times that).

A file that is clearly shorter than expected is truncated. That is the easy case. Two other outcomes need different handling:

• The size matches the original exactly but the file will not play. The transfer completed, so the problem is elsewhere: a format QuickTime does not support, or a file that was already damaged at the source. See how-to-check-whether-a-video-file-is-corrupted-on-a-mac.
• The size is 0 bytes. The copy created the file name and nothing more. There is nothing to salvage.

Why a half MP4 is worse than a half MKV

What can be rebuilt depends on the format, because formats differ in where they keep their index, the table that tells a player where every frame lives.

MP4 and MOV from most cameras and phones put the index at the very end of the file, because it cannot be written until all the frames are. A half-copied MP4 therefore has frames and no index. QuickTime opens the file, finds no index, and reports that it cannot be played. The frames are sitting there; they just need a new index built by scanning them. This is a standard repair.

Some MP4s are written with the index at the start (files prepared for web streaming, and exports from some editors). A truncated copy of one of those will play up to the cut in most players, because the index is present and only the tail is missing. You may not need a repair at all, though a repair will produce a cleaner file with an index that matches the actual length.

MKV and WEBM store their structure in clusters spread through the file. A truncated copy plays up to the cut in a tolerant player, then stops. Converting it to MP4 gives you a properly closed file.

TS and MTS (camcorders, broadcast captures) are built from independent packets and survive truncation best of all; the first half is simply a shorter video.

Which format you have is the extension, most of the time. If the extension does not match the contents (a file renamed from .mkv to .mp4 in transit, for instance), the Terminal check in the post linked above sorts it out.

Rebuilding the half you have

Work on a copy. Then:

1. Open Manatee and choose Repair.
2. Drag the truncated video into the window, or right-click it in Finder and choose Fix with Manatee.
3. Manatee scans the file for frame data, rebuilds an index that matches what it found, and writes a new file alongside the original. The original is never modified.
4. Read the outcome. For a truncated file the honest answer is almost always Partial, with a statement like “the first eleven minutes”. That is the video up to the point the transfer stopped, playable in QuickTime, on a phone, anywhere.
5. Open the result and scrub to the end to confirm it plays through.

Two cases where the outcome will be Not fixable instead: the transfer stopped before a single complete frame was written (the file is a few kilobytes), or the file is not actually a video but something renamed to look like one. Manatee cannot conjure frames that are not in the file, and it has no access to the source device, so it cannot fetch the missing half from anywhere. Data that did not make it across is gone.

If the truncated file is an MKV or WEBM that already plays up to the cut, you can skip Repair and use Convert to produce an MP4 of the part that exists. Either route works; Repair gives a clearer report of what was recovered.

Why transfers get cut off, and how to avoid the next one

The causes are mundane and mostly preventable:

• Ejecting too early. The Finder progress bar finishing does not always mean the drive has finished writing. Eject from Finder (drag to the Trash or click the eject icon) and wait for it to disappear before unplugging.
• Sleep. A Mac that goes to sleep mid-copy over Wi-Fi or to a network share can drop the connection. For large transfers, keep it awake (Energy settings, or a caffeinate command in Terminal).
• AirDrop interruptions. Moving the phone out of range, locking it, or switching apps can cancel an AirDrop. Keep both devices unlocked and near each other until the transfer completes.
• Full destinations. A copy to a drive that runs out of space fails partway with a file exactly as big as the remaining space was.
• 4 GB limits. A FAT32-formatted drive or card stops every file at 4 GB. Format the drive as exFAT or APFS for anything bigger.

After any important transfer, compare sizes on both ends. For files that really matter, run shasum on both copies in Terminal; matching output means the bytes are identical.

Questions

The half-copied video plays but the last part is missing. Do I still need to do anything?

Only if you want a clean file. A video that plays up to the cut has a usable index already. Running it through Repair or Convert writes a new copy with a duration that matches its actual content, which avoids the player showing the wrong length or stalling at the end.

Can Manatee pull the rest of the video from my phone or the cloud?

No. Manatee works on the file you give it and nothing else. It has no access to your devices or accounts, it does not go online except to check its license, and it does not recover deleted files. Getting the other half means finding another copy yourself, using the list at the top of this post.

A transfer was interrupted and now both the copy and the original look broken. What happened?

If you moved rather than copied, Finder may have deleted or partially written the source. Check the Trash and the source device first. Time Machine or the cloud service’s version history may have a copy from before the move. Going forward, copy, verify, then delete, rather than moving.