I intend to buy one more unit of DDR4 RAM. What is the recommendation? As I checked online, most of the RAMs are 2400. If I buy 8GB DDR4-2400, is it compatible with the current 16GB DDR4-2133? or do I have to buy 16GB DDR4-2400 instead of 8GB? Any problem if memory size is different? Please advise. Thanks.
If you will install different (in size, frequency or P/N) memory module, your computer will not work in double channel memory mode (i.e. lower performance). If you will install similar module, you have good chance that the computer will work in double channel memory mode. I say, you have good chance, because to ensure it, you should install memory kit (two identical memory module, chosen in manufacture). On ebay you may find memory you need: https://www.ebay.com/itm/Crucial-16GB-PC4-2133-Laptop-RAM-SODIMM-CT16G4SFD8213-Memory-100-Tested-Gra... . This is not endorsement to buy from this seller, but example only.
thanks for prompt response. It's difficult for me to buy from eBay. Is it possible to buy this model Crucial 16GB DDR4 2400Mhz Sodimm Value Ram - CT16G4SFD824A? Everything is compatible except it's 2400MHz. According to Crucial web site, it can support 2133MHz.