Allan Jones wrote:
It appears that the SDXC card slot was dropped from most MacBook Pro models made after 2015, If you need a built-in card slot in a new Macbook Pro , you must buy the more expensive 14- or 16-inch models.
Otherwise you need an external USB card reader.
That's true, but the OP said that he had a slot "on the right [hand side]" of his Retina MacBook. The question was whether you could put the same cards in both readers.
As far as I know, yes, although there might be some cases in which an older machine/OS would have problems with reading the contents of a particular card. Like if you formatted a card using APFS (instead of the default filesystems like FAT32 (SDHC) and exFAT (SDXC) that digital cameras expect. Or if you were trying to import Raw files, and an older system didn't have support for a particular Raw format (even though it could physically copy Raw files off the card).