Large ProPes export stuck at 99.8% & preview issue

[r3dfx080]

The fewer issues I have to read, the more new features I will have time to implement, so I ask that you please try these things first

• Try with a different kind of file to confirm that the problem is related to just this file.
• Try with the newest version from GitHub
• Read the documentation and FAQ, Known issues, Troubleshooting
• Search for your problem under Issues or Discussions

Steps to reproduce

• Record a large file using AJA Control Room
• Try to export a cut version and check the preview

Expected behavior

An exported version should appear playable and with correct tags. A preview should be displayed in LosslessCut

Actual behavior

The export itself becomes stuck at 99.8%, these tags are written during export. This only happens on large files (~20GB+)

General
Format                      : MPEG-4
Format profile              : QuickTime
Codec ID                    : qt   0000.02 (qt  )
File size                   : 25.1 GiB

The preview is unavalaible

Using different FFmpeg .exe solves preview issue, but the export is still appears to be non-functional

Provide an error report

No error occurred.

App state:
{
"err": false,
"state": {
"lastAppVersion": "3.67.2",
"captureFormat": "jpeg",
"keyframeCut": true,
"preserveMetadata": "default",
"preserveMetadataOnMerge": true,
"preserveMovData": true,
"preserveChapters": false,
"movFastStart": true,
"avoidNegativeTs": "make_zero",
"autoMerge": false,
"timecodeFormat": "timecodeWithFramesFraction",
"invertCutSegments": false,
"autoExportExtraStreams": true,
"askBeforeClose": false,
"enableAskForImportChapters": true,
"enableAskForFileOpenAction": true,
"playbackVolume": 0.19,
"autoSaveProjectFile": false,
"wheelSensitivity": 0.2,
"waveformHeight": 40,
"language": "en",
"ffmpegExperimental": false,
"autoLoadTimecode": false,
"autoDeleteMergedSegments": true,
"exportConfirmEnabled": true,
"segmentsToChapters": false,
"simpleMode": false,
"cutFileTemplate": "${FILENAME}_${SEG_NUM}${EXT}",
"keyboardSeekAccFactor": 1.03,
"keyboardNormalSeekSpeed": 1,
"keyboardSeekSpeed2": 10,
"keyboardSeekSpeed3": 60,
"treatInputFileModifiedTimeAsStart": true,
"treatOutputFileModifiedTimeAsStart": true,
"safeOutputFileName": true,
"enableAutoHtml5ify": true,
"segmentsToChaptersOnly": false,
"enableSmartCut": false,
"storeProjectInWorkingDir": false,
"enableOverwriteOutput": true,
"mouseWheelZoomModifierKey": "ctrl",
"mouseWheelFrameSeekModifierKey": "alt",
"mouseWheelKeyframeSeekModifierKey": "shift",
"segmentMouseModifierKey": "shift",
"captureFrameMethod": "videotag",
"captureFrameQuality": 1,
"captureFrameFileNameFormat": "timestamp",
"enableNativeHevc": true,
"enableUpdateCheck": false,
"cleanupChoices": {
"trashTmpFiles": true,
"askForCleanup": true,
"closeFile": true,
"cleanupAfterExport": false,
"trashProjectFile": true
},
"allowMultipleInstances": true,
"darkMode": true,
"preferStrongColors": true,
"outputFileNameMinZeroPadding": 1,
"cutFromAdjustmentFrames": 0,
"cutToAdjustmentFrames": 0,
"storeWindowBounds": true,
"thumbnailsEnabled": false,
"keyframesEnabled": true,
"reducedMotion": "user",
"filePath": "I:\test.mov",
"fileFormat": "mov",
"externalFilesMeta": {},
"mainStreams": [
{
"index": 0,
"codec_name": "prores",
"codec_long_name": "Apple ProRes (iCodec Pro)",
"profile": "Standard",
"codec_type": "video",
"codec_tag_string": "apcn",
"codec_tag": "0x6e637061",
"width": 720,
"height": 576,
"coded_width": 720,
"coded_height": 576,
"has_b_frames": 0,
"sample_aspect_ratio": "59:54",
"display_aspect_ratio": "295:216",
"pix_fmt": "yuv422p10le",
"level": -99,
"color_space": "smpte170m",
"color_transfer": "bt709",
"color_primaries": "bt470bg",
"field_order": "tb",
"refs": 1,
"id": "0x1",
"r_frame_rate": "25/1",
"avg_frame_rate": "25/1",
"time_base": "1/25000",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 276491000,
"duration": "11059.640000",
"bit_rate": "45875200",
"bits_per_raw_sample": "10",
"nb_frames": "276491",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0,
"non_diegetic": 0,
"captions": 0,
"descriptions": 0,
"metadata": 0,
"dependent": 0,
"still_image": 0,
"multilayer": 0
},
"tags": {
"creation_time": "2026-01-10T10:38:47.000000Z",
"language": "eng",
"handler_name": "Apple Video Media Handler",
"vendor_id": "appl",
"encoder": "Apple ProRes 422",
"timecode": "00:00:00:00"
},
"side_data_list": [
{
"side_data_type": "Frame Cropping",
"crop_top": 0,
"crop_bottom": 0,
"crop_left": 8,
"crop_right": 9
}
]
},
{
"index": 1,
"codec_type": "data",
"codec_tag_string": "tmcd",
"codec_tag": "0x64636d74",
"id": "0x2",
"r_frame_rate": "0/0",
"avg_frame_rate": "25000/1000",
"time_base": "1/25000",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 276491000,
"duration": "11059.640000",
"nb_frames": "1",
"extradata_size": 38,
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0,
"non_diegetic": 0,
"captions": 0,
"descriptions": 0,
"metadata": 0,
"dependent": 0,
"still_image": 0,
"multilayer": 0
},
"tags": {
"creation_time": "2026-01-10T10:38:47.000000Z",
"language": "eng",
"handler_name": "Time Code Media Handler",
"reel_name": "Untitled",
"timecode": "00:00:00:00"
}
},
{
"index": 2,
"codec_name": "pcm_s16le",
"codec_long_name": "PCM signed 16-bit little-endian",
"codec_type": "audio",
"codec_tag_string": "sowt",
"codec_tag": "0x74776f73",
"sample_fmt": "s16",
"sample_rate": "48000",
"channels": 2,
"bits_per_sample": 16,
"initial_padding": 0,
"id": "0x3",
"r_frame_rate": "0/0",
"avg_frame_rate": "0/0",
"time_base": "1/48000",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 530862720,
"duration": "11059.640000",
"bit_rate": "1536000",
"nb_frames": "530862720",
"disposition": {
"default": 1,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0,
"non_diegetic": 0,
"captions": 0,
"descriptions": 0,
"metadata": 0,
"dependent": 0,
"still_image": 0,
"multilayer": 0
},
"tags": {
"creation_time": "2026-01-10T10:38:47.000000Z",
"language": "eng",
"handler_name": "Apple Sound Media Handler",
"vendor_id": "[0][0][0][0]"
}
}
],
"copyStreamIdsByFile": {
"I:\ivan_tape_1.mov": {
"0": true,
"1": false,
"2": true
}
},
"cutSegments": [
{
"start": 0,
"end": 11059.64
}
],
"mainFileFormatData": {
"filename": "I:\ivan_tape_1.mov",
"nb_streams": 3,
"nb_programs": 0,
"nb_stream_groups": 0,
"format_name": "mov,mp4,m4a,3gp,3g2,mj2",
"format_long_name": "QuickTime / MOV",
"start_time": "0.000000",
"duration": "11059.640000",
"size": "65547515928",
"bit_rate": "47413851",
"probe_score": 100,
"tags": {
"creation_time": "2026-01-10T10:38:47.000000Z"
}
},
"rotation": 360,
"shortestFlag": false,
"effectiveExportMode": "separate"
},
"appPath": "C:\Users\_redfox\Desktop\LosslessCut-win-x64\resources\app.asar",
"platform": "win32",
"arch": "x64",
"version": "3.67.2"
}

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Maybe related #2594

[mifi]

I've fixed the preview issue. The export stuck at 99.8% sound similar to #2594 but i'm not sure. Does it happen with all files of this size or just one? what about smaller files?

[r3dfx080]

I've fixed the preview issue. The export stuck at 99.8% sound similar to #2594 but i'm not sure. Does it happen with all files of this size or just one? what about smaller files?

So far I've tried multiple files with same results... As far as I remember, this wasn't a ProRes-only issue since I couldn't export large DV files too

[mifi]

I just generated a 30GB PRORES file and exported the whole file unedited using LosslessCut. And it got stuck at 99.9% for about 3 minutes. Then it finished. How long did you wait? What if you go to to "Last ffmpeg commands", find the most recent command that appearaed after you started your operation (the one that gets stuck at 99.8%) and then you run that command from the command line (outside LosslessCut) using ffmpeg.exe, does it get stuck there also? Another thing to try is to disable "MOV Faststart" and make sure all slow options like "Preserve all MP4/MOV metadata" are off.

[r3dfx080]

At first I waited for about 10-15 minutes with ~50GB file. The point is: it wrote the entire file (judging by the size) and then stalled at 99.9%... Unfortunately I won't be able to replicate my exact issue until 6th of February since I'm in the different city rn, away from my PC with all AJA captured footage
I'll try and record a ProRes file with OBS, however I'm not sure whether it will behave the same way

[mifi]

Oh that's too bad. I'd really like to get to the bottom of this issue, but I cannot reproduce the problem (I think).
I believe one reason why it can write the whole data, and then get stuck at 99% is that it will first copy all the raw data, then later (especially if enabling flags like "MOV Faststart"), it will go back and place the faststart moov atom at the beginning of the file. There may also be other things that get written at the very end (this is all handled by ffmpeg).