first of all having the old drive still booting will be silly because you want your C drive on the faster SSD... i know reinstalling Windows sounds like hassle but it honestly is worth the effort
now bg3, in the early days stored its saves in documents/ Larian but Microsoft fucked up everyones documents by moving them into their onedrive folders to try and trick people into using that ... Larian moved save games to the local folders instead from about a year ago
C:\Users\%username%\AppData\Local\Larian Studios is the current path to saves where C is the default Windows location for it and %username% will be different for each person with an account on the system
yes you can change these locations for example mine is G:\Users\Ussnorway\AppData\Local\Larian Studios but most people are better to just leave things in their default paths
