Large arrays warning when changing file modified date

[RadicalDad]

I get this warning message when using ExifTool (via ExifToolGUI) on some DNG files when changing the file date modified to Date Taken.

Warning: [Minor] Not decoding some large array(s). Ignore minor errors to decode

The DNG files in question were created by Lightroom's HDR merge.  Oddly, only some DNGs created this way generate the warning.

My concern is whether I have somehow damaged the metadata or the "large arrays" in the DNG file by using ExifTool to change the file modified date.  I don't want to find out years from now when I try to use the DNG for something else and discover it has been corrupted by this process.  I suppose I should be curious as to what the error message really means, but in truth, I am not.  Thus, don't waste your breath, your typing fingers, and the electrons trying to explain the guts of the error message to me.  Just let me know if I can ignore it, or if I need to do something else to properly preserve the file in this case.

Thanks   

And thanks for this great tool!  Has made my life soooo much easier.

[StarGeek]

The DNG files in question were created by Lightroom's HDR merge.  Oddly, only some DNGs created this way generate the warning.

My concern is whether I have somehow damaged the metadata or the "large arrays" in the DNG file by using ExifTool to change the file modified date.
...
Just let me know if I can ignore it,

You can ignore it.

I suppose I should be curious as to what the error message really means, but in truth, I am not.

Too bad, I'm going ahead and explaining, if not for you, then for someone else who may come across this question.

Lightroom keeps track of every edit you do to a file and saves this process, I believe in the XMP-xmpMM tags.  If you do a lot of edits, this can lead to a huge amount of data added to the file.  So much that if exiftool tried to extract it all, it would significantly increase the processing time.  Since this data is only meaningful to the editing program and largely useless to actually read, there isn't much reason to list it all.  So exiftool issues this warning and tells you how to extract it (use the -m (-ignoreMinorErrors) option) if you really want to.

I believe that it is a common complaint on the Adobe forums about how much data their programs add to the files.

[RadicalDad]

Gah!  I knew somebody would explain it!  And then I took the time to read the explanation!     Thanks!

I don't need the edit history as all I did was combine the files using the default HDR settings in Lightroom.  I can understand some other operations possibly needing to read and/or decode all that information, but just changing the file date to match date taken shouldn't have to bother with any of that.  Does ExifTool rewrite the entire file during this process, dropping the excess information along the way?

I have a new camera and can now shoot JPEG+RAW.  It is my first camera capable of saving RAW files, and I am playing around with its capabilities.  I think the DNGs that have the problem were the HDR DNGs created with RAW files.  The HDR DNGs created with the matching JPEGs didn't generate this warning when I changed the file dates.  I'm guessing there must be more data to sling around with DNGs created from RAW files.

[StarGeek]

Does ExifTool rewrite the entire file during this process, dropping the excess information along the way?

Exiftool always rewrites the entire file, adding and removing only that data which you specifically tell it to add/remove.  If the new file is successfully created, it then renames the original by add _original to the end and renaming the new file to the original name.  See the second paragraph under Description in the docs and also the -overwrite_original option.  FAQ #13 also explains why the size of the file might change when editing.