Installing Mojave on a volume in a RAID 1 external HDD

I own a WD MyBook Duo External HDD that is 16TB (2 x 8TB physical volumes). I set it up as an 8TB RAID 1 - Mac OS Extended (Journaled). I then partitioned the 8TB RAID 1 into 2 x 4TB volumes using Disk Utility. I call one volume Time, the other volume Dave.

Time is being used for backing up my 3TB iMac using Time Machine. The other volume, Dave, is free to do what I want with. Dave currently holds only 110MB or so.

I have some games on Steam that have stopped working since updating to Catalina. So installed Mojave (and Steam) on a virgin 64gb USB memory stick and currently boot from that whenever I want to use Steam for those games that no longer work with Catalina.

This has been working okay for a while now but of course I don't have much room on the device it to play with. So I'm considering installing a bootable copy of Mojave on the volume I call Dave so that I may access the 4 TB of free space on it.

I read somewhere that downloading Mojave and pointing the installer at a chosen volume will wipe the said volume. So, if I want Mojave on Dave, I will need to move the small amount of stuff already on it elsewhere first.

What I want to check however is that the Mojave installer will only wipe Dave and not Time as well. I suspect that it will only wipe Dave but I want to be sure in case putting a bootable copy of Mojave on Dave screws up my Time Machine volume.