TL;DR: Recently during a bulk move of list items from one list to another, many list item attachments didn't get moved with the items. I'm wondering what can cause this. The SharePoint 2010 Content and Structure move process is not very transparent and there are no logs that I know of.
Context: We have two high volume lists: one which is active and one which is an archive of items from the original. Because of the volume of data and SharePoint's performance on large lists, we periodically move inactive items from the base list to the archive.
Problem: On a recent move using SharePoint's Content and Structure tool, the items successfully moved from the base list to the archive list, but many (1000s) came over without their original attachments. Because the tool does not retain the items in the original list, those attachments are now all gone.
Need: While recovery would be ideal (I just don't see a possible way to recover it as we no longer have database backups since this wasn't discovered immediately), I am hoping to at least identify what causes this data loss, so we can mitigate it in the future.
Ask: SharePoint experts, you're my only hope! Help me determine why attachments aren't being moved with their list items using the Content and Structure tool.
If the structure of the list is not too complex and you don't have any workflows attached to the list, I'd suggest you make your own migration script in PowerShell.
The script would iterate throuh items to be moved, create similar items in the target list and then also move attachments.This way, you could choose if you want to immediately delete the source items or not, make temporary backups of attachments on a local drive, etc.