In case it's of any use to anyone, I've come up with a bit of a workaround to the problem outlined above. This will only work if you have sufficient space on your primary drive to download at least one set of additional content/Jam Packs at a time because it involves downloading them as per usual, moving to the second (larger) internal disk and then creating symbolic links between the two.
Here's a basic step-by-step:
1. Download as much additional content within MainStage as you have space to do.
2. Once downloaded you'll need to find out where they are stored. For me, it turned out that all the sample files I needed were stored in "/Library/Application Support/Logic/EXS Factory Samples" and "/Library/Application Support/GarageBand/Instrument Library/".
3. Copy these two folders to the second hard disk. I put them within their own subfolder on my second disk called 'Logic and GarageBand data'.
4. Create symbolic links to your moved folders. For this you will need to go into the Terminal, cd to the folder where you have made your copies of the GarageBand and Logic folders and type the following:
ln -s "/Volumes/<NAME OF SECOND DISK>/<FILE PATH TO YOUR COPIED FOLDERS>/Instrument Library/" "Instrument Library"
ln -s "/Volumes/<NAME OF SECOND DISK>/<FILE PATH TO YOUR COPIED FOLDERS>/EXS Factory Samples/" "EXS Factory Samples"
Substitue in the correct names and paths where I've put <NAME OF SECOND DISK>/<FILE PATH TO YOUR COPIED FOLDERS>
This will create two symbolic links - one called 'Instrument Library' and one called 'EXS Factory Samples'. Your Mac will see these symbolic links as if they were the original folders themseslves. So, what will happen is that your copy of MainStage will look for an instrument sample in the folder /Library/Applicaiton Support/GarageBand/ and see the symbolic link called 'Instrument Library' instead of the folder 'Instrument Library' that it is expecting. This won't matter as it won't actually realise it's seeing a symbolic link instead of a folder and, instead, will carry on as if the folder was there. By this point, the underpinnings of OSX will point MainStage to the linked-to folder on your second disk where all the sample files are. MainStage will load the samples and be none the wiser to the fact that you have moved them and reclaimed all that space on your primary disk!
5. Erase the 'Instrument Library' folder from /Library/Applicaiton Support/GarageBand/ and copy the 'Instrument Library' symbolic link from your second disk to that folder. Do the same for the 'EXS Factory Samples' folder within /Library/Application Support/Logic.
6. Empty your trash to relciam that space!
7. If everything has gone to plan, MainStage should open without hitch and start using your samples from their new location. If you previously had your samples on an SSD (as I did) you will definitely notice the extra time it takes to load them off HDD folllowing the move but once they're loaded it doesn't seem to affect performance in live use.
I hope that all makes sense. If you find it helpful then you can use the same technique to move any files and folders you like from the primary SSD to a slower, but larger second HDD to make room for your more day-to-day files. A good example might be your iTunes library, which really doesn't require instant access, or else your iPhoto library which can get very large and, agagin if like me, isn't used very much it might as well go on a slower disk and save you room for the items you need quickly.
