As of Xcode 11 (released to beta to developers yesterday), we should locate the simulator runtimes located in /Volumes/*/Library/Developer/CoreSimulator/Profiles/Runtimes
I suggest you move them to that location on your spare drive. Doing so while CoreSimulatorService is running will probably confuse its state because we don’t currently handle runtimes vanishing from the root filesystem (we do handle them disappearing due to a disk eject through), so you will probably need to SIGKILL CoreSimulatorService after doing the move by running:
sudo killall -9 com.apple.CoreSimulator.CoreSimulatorService 2> /dev/null
If you don’t want to wait for Xcode 11, you can also just create a symlink from /Library/Developer/CoreSimulator/Profiles to /Volumes/MyHDD/Library/Developer/CoreSimulator/Profiles.