I've had my iPhone shutdown on me a few times while recording using DJI GO app and my OSMO Mobile.
The App said the footage was GONE, but that is NOT TRUE!  I've found a way to recover the data using iTunes and a computer, but no way to delete the files on my iPhone.  
Hey DJI GO developers, iOS users need a way to manually manage the HG300Storage folder data!  Your app may say the library is empty when there are video files.
I have 1.5GB of footage stuck on my phone that I cannot delete because your app won't show it.   Please consider making an update to allow us to find and delete footage in the hg300storage folder.

I made a video about this to help others recover lost footage and to get some awareness of the issue.

Really useful video, thanks for taking the time to create and share it.  I have a similar file that currently sits at about 6.2Gb when I only have 2 clips totaling about 15 seconds in the app. The majority is a timelapse that failed and which doesn't appear in the app. Have you reported this to DJI support so that they can look at a solution?

I have found a couple of programs that seem to allow you to delete the files using a computer - iExplorer (paid for) and iFunbox (free)

@DJI-Ken, could you help by getting this fast tracked to the R&D team for the app?

A1000 Posted at 2017-1-22 00:30
Really useful video, thanks for taking the time to create and share it.  I have a similar file that currently sits at about 6.2Gb when I only have 2 clips totaling about 15 seconds in the app. The majority is a timelapse that failed and which doesn't appear in the app. Have you reported this to DJI support so that they can look at a solution?

I have found a couple of programs that seem to allow you to delete the files using a computer - iExplorer (paid for) and iFunbox (free)

Thanks for a reply.  I didn't contact DJI support, I thought that's what this forum was for.  I'm sure many people have videos stuck on their iPhones that they cannot delete.   Your instance of a 6+GB timelapes is likely a common occurance.  There seems to be no  "emergency" auto stop rec in the app when battery is running low.  The files are there and are usable (not corrupt, no recovery apps needed), but the app won't show them.

@DJI-Ken or other app developers.  We iOS users need a way to "rescan for videos" inside the app and have it show any MOV file with data inside the hg300storage folder. Or give users write access or a "clear all storage" option in DJI GO settings.   We can recover the "lost" footage using itunes, but we have no way of deleting the files to free up precious gigabytes!

I found a way to DELETE YOUR LOST AND HIDDEN FOOTAGE stuck inside DJI GO hg300 storage folder without having to delete the whole app.  You can now REGAIN PRECIOUS iPHONE STORAGE SPACE!
Here's how to do it:

Netcruzer Posted at 2017-1-22 21:08
I found a way to DELETE YOUR LOST AND HIDDEN FOOTAGE stuck inside DJI GO hg300 storage folder without having to delete the whole app.  You can now REGAIN PRECIOUS iPHONE STORAGE SPACE!
Here's how to do it:
https://youtu.be/f5uceigdYA4

For me when I followed your video, by deleting the whole folder it also deleted those videos that were showing in the app and could be deleted.

I have found a couple of programs that seem to allow you to delete the individual files using a computer - iExplorer (paid for) and iFunbox (free).

A1000 Posted at 2017-1-23 01:04
For me when I followed your video, by deleting the whole folder it also deleted those videos that were showing in the app and could be deleted.

I have found a couple of programs that seem to allow you to delete the individual files using a computer - iExplorer (paid for) and iFunbox (free).

Yes, it's an all or nothing approach. iTunes only allows me to delete the whole folder not pick and choose which files to keep or delete within it.   To get individual file access inside hg300 storage folder in DJI GO for iOS you would need to buy a PC or MAC program, like iExplorer or Phoneview.

I'm OK with the free way for now. Just saving out the folder then deleting it all from time to time when I notice it getting large if my library is empty.

iFunbox is free and slightly less polished UI but works and allows you to delete individual files, but you do need a computer as you say.

Here I recommend a free trial SD card recovery software for you. Cisdem DataRecovery is pretty easy to use. Its user-friendly interface makes everything clear and easy to operate. Just download the free trial version to recover data from SD card.

Bianca96000 Posted at 2017-3-15 02:00
Here I recommend a free trial SD card recovery software for you. Cisdem DataRecovery is pretty easy to use. Its user-friendly interface makes everything clear and easy to operate. Just download the free trial version to recover data from SD card.

I don't think SD recovery software works for iPhones/iOS users, but others might have luck with it.
Does the folder get re-created automatically by the DJI GO app after it is deleted in iTunes?

Maybe you can try some free trials of data recovery software if you want to recover lost files without iTunes.