I usually choose the Routable Bicycle (openfietsmap lite) option as that seems to work best with Garmin Edge GPS. When it comes to the download page, I download the compressed file containing the single image which I then unzip and place in the Garmin directory of a micro SD card.
I add countries to that directory as I need them. I started with UK and Spain then added Holland and Germany (so far). Had no problems doing it this way.
Hope this helps.
Did you just copy the img file across to the SDcard?
I have a microSD card for an etrex30 which I load via BaseCamp.
But having just got myself the OVMlite of all of France & to Barcelona via the Basecamp .zip and .exe options, the .exe won't let me rename the .img file and uninstalled a previous .exe version which was my UK/Eire map. (It ignore my Benelux map leaving it as is, but that did arrive with a unique name when I got my download).
The uSD card has enough space for all 3.
I can work around it for now because I only need France for a club Raid Alps for next week or so, then for PBP, then I can re-install UK from scratch and ditch France, but it would be nice to have 3 or 4 mapsets to save re-installing over again, if I can. UK, Flanders, Mallorca are all frequent trips and a Mosel trip coming next year would all be nice to have, without inflating to an "all of Europe" monster.