Yes this was a big problem. Daedalus just ate up my SSD while I have a mostly free 2TB HDD sitting doing nothing.
It turned out to be very simple though. Here are the steps for Ubuntu based systems;
Make sure that Daedalus is not running.
Copy the ~/.local/share/Daedalus directory somewhere in your harddisk. In my case I copied it to /mnt/P300/Daedalus.
To back up, I renamed the original like ~/.local/share/Daedalus_
Create a symlink in ~/.local/share to point to /mnt/P300/Daedalus like
ln -s /mnt/P300/Daedalus ~/.local/share/Daedalus
We are almost finished. Open ~/.local/bin/daedalus-mainnet with an editor. There are two lines starting with exec. In both of them add -m /mnt:/mnt before -m /home:/home. In my case /mnt is where my HDD is mounted. You should first check the mountpoint of your HDD. Perhaps you need to add -m /media:/media or whatever your mountpoint is.
Finally open ~/.local/share/Daedalus/mainnet/namespaceHelper. (now this file is in fact in the HDD since you have already linked at step 4) with an editor and repeat exactly what we did in step 5.
Say some prayers.
Now you are probably OK and when run, Daedalus should start working on the HDD copy.
Make sure it finalizes replaying the ledger and synching the blockchain.
Make sure everything works just fine.
You may now delete the backup ~/.local/share/Daedalus_ from your SSD.