Not currently possible, and it's a tough problem to solve, because not all changes you would make to an item in one library (notes, tags, etc.) would necessarily be appropriate to copy between libraries. But we're planning to at least offer better ways to keep items in sync manually (e.g., so that dragging items again will update the items in the target library instead of skipping them).

There's no limit, but you'd very likely run into trouble trying to drag 11,000 items. For libraries that large, we generally recommend just maintaining a single version as a group library rather than trying to keep multiple libraries in sync.

But you can also just drag in batches. Since Zotero won't duplicate items in the group, there's no harm even if you accidentally drag something more than once.

I'd be curious to see a Debug ID for an attempt to drag 11,000 items that didn't work, though. In principle it should work, and it's possible it's either just taking a long time with no progress indicator or something is timing out in the background. (One change we could make here would be to show a progress meter when dragging many items.)

Dan, I'm spending a lot of time keeping My Library and some of my Shared Libraries in sync, I'm never quite sure if the two are actually synchronized. It affects the quality of the work we can do, in that we can easily miss key references if we don't sync well. I realize from the text above that solving this is non-trivial. Here is a relatively simple solution that wouldn't make synchronizing completely automatic but would save time and improve the work we can do with Zotero. The solution would be to add a folder called "Library Comparisons" at the bottom of the My Library folder structure, and inside that folder you would have subfolders for each My Library to Non-My Library comparison. Each of these would be empty, so that it takes no bandwidth unless the user wants to do the comparison. If the User chooses one of the comparison folders, and right clicks and selects "Do Comparison" it would run the comparison, and create three results: In Both: X number, In My Library but not B library: here it would list the actual references, In B library but not My Library: also list the references. The User would then be able to manually choose which items to sync and where by right clicking an item and choosing which folder or root directory it should go to, in the comparison library. Let me know if this is doable, I would be happy to test it if you try it.

I am also interested in finding a way to keep my personal and my group library in sync. Is this something that is being considered for future releases?

I just set up a group library for collaboration with a co-author. As I collect more references for our book, I thought I could maintain my own reference entries in my personal library and they would also update in the group library. Now I am not sure how to handle updating them in two places... Grateful for any tips!