How are you with Unix CLI Commands with the umbrelOS?
You would need to either mount the 2nd drive where you want the storage to be, like I do:
/dev/nvme1n1 1.8T 890G 851G 52% /home/umbrel/umbrel/app-data/bitcoin
Or use a symbolic link to get the data where you want it. Which I do for some other apps:
umbrel@umbrel PROD:~/umbrel/app-data$ ls -l ~/umbrel/app-data/retro-mike-*
lrwxrwxrwx 1 umbrel umbrel 72 Jan 23 22:56 /home/umbrel/umbrel/app-data/retro-mike-bc2-node → /home/umbrel/umbrel/app-data/bitcoin/retro-mike-apps/retro-mike-bc2-node
This would mean you install umbrelOS on the Primary Drive and hope it is not too small to run things!
NOTE: These changes will NOT survive an update from Umbrel!!!