I'm not sure if anyone else has done this yet, but I upgraded the internal SSD for my series S with one from a broken series X I purchased on ebay.
If you want to do this swap, you'll need to clone all of the partitions from the original 512gb ssd to the 1tb ssd with the exception of the user content partition. If you put the ssd in straight from the series x, it won't power on.
Also if you want to use your old 512gb ssd as an expansion card just buy one of the cfexpress to...
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Source: https://gbatemp.net/threads/series-s-in ... de.610226/