Help! Massive mobile downloads.lrdata

Everything I look up is about Lightroom Classic.

That's because "Mobile Downloads.lrdata" is ONLY created and used by Lightroom Classic. It's the default location where a sync-enabled Classic stores the images which sync down from the cloud, so if it's "still growing" that would suggest that you are still using Classic as well as the Lightroom Desktop app. That's not an issue, but you do need to understand that every image that you import into any of the Lightroom cloud apps will also download into your synced Classic catalog and will be stored in that default location.

You can do several things:

1. In Classic you can move the images out of that folder into a different folder on a different drive, using drag and drop in the Classic Folders panel. But new images would still download into the default folder.

2. In the LrClassic Preferences>Lightroom Sync tab you can change the default location for sync downloads to a different folder (e.g. on an external drive). Note that when you do that, LrClassic will attempt to move all the existing images from Mobile Downloads.lrdata to the new specified folder, but I think that initial move will store the images in a single folder (you can specify that subsequent images are downloaded into one of the standard date-based folder hierarchies, but that doesn't apply to the inital move).

3. If there are thousands of images to deal with in the existing Mobile Downloads.lrdata you might want to do a combination of 1 and 2, i.e. using drag and drop in the Folders panel, drag the images piecemeal into a preferred set of folders on an external drive, then when that's complete change the default location to a different drive.

The Mobile Downloads folder is where, if you've ever used Classic and sync was turned on, any photos in the Cloud would have automatically download to.

Does that sound feasible, that you have used Classic in the past?

Just to confirm, where is the folder located on your HD?