similar issue, when I installed the second 2.5" SSD (Crucial MX100 512GB) as the second SSD in my recently purchased 7577, the bios can find it fine. More detail about it , the second SSD has installed UEFI-booted windows 10 system in it. And it can be booted fine. However, when I try to wipe the SSD off and install windows 10 freshly via UEFI-booted USB win10 disk, Windows 10 installer just CANNOT find the SSD. DELL tech, please keep us updated about the solution for this issue. Thanks!
updated bios to 1.3.2 for 7577, which is released today(12/21). The new bios fixed the 2nd SSD detection bug in Windows10 installer. Thanks DELL for the update!