@thegr8mage Please open Disk Utility (in Applications > Utilities), click the name of your external drive in the left panel, and list the mount point you see there. It should say LaCie since that's what you've named your drive, but if you see LaCie 1 (or higher), that's the issue.
In that case, eject the drive in software by right-clicking it, disconnect the drive, open a Finder window, click cmd-shift-G, and enter /Volumes in the search field. You'll see your Mac's internal drive and perhaps one or more folders with LaCie in the name. Delete those, restart your Mac, reconnect the drive, and see whether you can download your new packs.
If this isn't the issue, please let me know whether you can actually play Sims 4 as it currently stands.
