In one day I noticed many issues with this software and will never use it again such as 1/3 write speed over SMB, failing fusemounts which needed "fuzemount -uz /mnt" for force unmount then issue came back and I read it's not maintained. Using mergerfs now which I look forward to being able to set drives as RO and also, SMB reported correct free space immediately.
It took me a minute to figure this out, but I ended up using it/liking because of it's simplicity. Basically you have to mount each HDD say at "/mnt/sdb1" etc. Then you can union "/mnt/sdb1 & /mnt/sdc1" to "/mnt/largepool" and access everything under "largepool". You can also just access things under /mnt/sdb1 or check where it's saving things etc. It saves files basically in round robin using the disk w/ the most free space. If you lose a disk, everything on the other disk is the same/fine etc.
Create Virtual Union: (Note the trailing slash!)
mhddfs is nothing fancy, create first, confirm working, you can mount/unmount/create/recreate THEN put in fstab and "mount -a"
Again, watch for trailing "/"