Galaria Jardin on Florida Street, just north of the McDonalds. Bunch of different electronics stores with all sorts of brands and types of external drives, just make sure you're clear with whether you want a HDD/SDD, which USB standard and speed, etc.
Did you try to contact any of the ML sellers directly? Or look up their information? Ar least some of them will have physical shops, and some will also deliver against COD. Just checking briefly, I found one of them with a shop right in the centre of BA: https://www.grapesmarket.com.ar/ No idea if it fits your requirements, but I'm sure you can find more.
Yes, this. I use Mercado Libre a LOT and you will see that most of these places have physical stores. Just look at the name and Google it or contact them and they will tell you. I ordered a bunch of artwork for my apartment and just did that with many of them. Same thing with a bunch of other things.
