WAT
LOL
that's like a friend of mine was trying to install Windows 10 on his work laptop today and it wouldn't install on the SSD (kept insisting to install on the other drive)
I can't recall why but it was ... fussy
It seems to be a known issue when trying to select the "customize this folder" from inside your user folder. It seems also to be the only folder with that option, other regular folders don't.
Windows is VERY picky about drive order and similar things. If the drive you want to install on isn't the first, you HAVE to have space on the first drive for it to install its magic "system" partitions.