To clarify what happened:

(1) Plug in an SD card with a folder full of photos
(2) Drag-drop the folder from the SD card to Drive folder in Files app
(3) Open drive.google.com
(4) Copy the folder that was copied to a Team Drive
(5) Delete the original folder

Now the files are permanently lost.

Specifically, the files weren't uploaded yet but it was hard for me to know that without checking the file counts manually.

It seems like there are at least three possible UX changes that could help prevent this:
1) Don't delete anything that hasn't been uploaded
2) Add a trash if anything local-only is deleted
3) Give a visual indicator that a folder is still uploading like we do on other OSes

Weifang, can you confirm our plans for these suggestions? I'm pretty sure we're working on 1 with the new Drive sync and 3 with the visual indicators work. Are there plans to add a Trash to ChromeOS?

To confirm - Improving our visual indicators on the file progress/activity is definitely on our roadmap. By adding more UI around this, it may introduce an additional confirmation dialog in the case where a user attempts to remove files that are not in a completely uploaded state.

Currently, there are no plans to add a Trash to Chrome OS.

Thanks for the details.  

To clarify, the case of files getting deleted is that the folder is removed from the web UI, and the sync process (not the user) deletes the local files, which IMHO should not happen without confirmation.  It sounds like the UI you're suggesting would be to prompt before allowing the user to delete the files locally, which is yet another case.

Will revisit when DriveFS lands, it will internally move things to trash so the semantics will be different.

Thanks for the update - that seems like it would be a reasonable experience for my situation.

This issue is no longer applicable under the new sync engine, even if you delete it still uploads the files so it can trash them rather than straight delete them.

This is great news, thanks!