You have 2 memory slots and the laptop uses DDR4-2400 SO-DIMM that will downclock to 2133 speed. You have an 8 gig module and an open slot. The Manual is incorrect saying that the memory is not accessible. It is. Also the maximum is 32 gigs not 12. It can run a pair of 16 gig modules. The storage is NVME M.2 SSD. There is no hardware limit but the largest M.2 SSDs you will easily find are 2 TB. You might find a 4 TB if you look hard enough. If so you can install as large as you can find and a 512 or 1 TB is not a problem at all.
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