Regarding Biab, I never had any problems with RTs and RDs on a separate drive.
In my case, the issue was with the Plugin, which seemed to be lost (it couldn't generate anything, in both Reaper and Studio One). I admit that I was also using 4 Symlinks for the main folders (Demos, Soloists, Songs and Lessons, and Styles), as I thought it would save time by launching everything from my C: drive.

Since I deleted everything and did a simple installation only on the 2025 SSD, the plugin is working fine. Is it related? Probably.
For everyone whose separate drive is working, don't change anything.

Simon rightly pointed out that working on external drives without explicitly assigning fixed letters could also cause a loss of bearings in Biab, particularly with Symlinks, but I had specifically assigned fixed letters to my drives.

Biab Audiophile 2025 11.28 Win