Are you on El Capitan? It changes permissions on things and there could be driver incompatibilities getting in the way as well.
I would suggest COPYING the photos from the LaCie to the Drobo using non-LR methods (i.e. Finder) then moving the source folders on the Lacie to another top-level folder called PhotosCopied or some such, then do a Locate Missing Folder for each top-level folders you’ve moved and select the new folder on the Drobo. Once you’re sure everything is working on the Drobo then delete the old source duplicates in PhotosCopied.
I am suggesting using COPY with a delete much later instead MOVE in case something goes haywire in the process. In previous versions of LR I’ve seen reports of photos entirely disappearing when moving w/in LR when they are on external drives. There must be some driver incompatibility where LR thinks the copy worked but it didn’t so it deletes the source file. Maybe that’s how things worked in the past, and now the error message is occurring and the source remains intact (hopefully)?
I am on El Capitan. I'll try your suggestion. Thanks.
Well that was just too simple! Thanks. I've been struggling with this for a couple hours. Appreciate your help!