If your SSD is low on space, then move your Bridge Cache to the HDD.
In Bridge.. Edit/Preferences/Cache/Location...
And then copy the existing cache from the SSD to this location and delete it from the SSD.
Close and re-launch Bridge.
Make the Cache Size appropriate for your image library. You can use one of two strategies:
1. Make the cache size small (minimum) and then check the "Automatically Export Cache to Folders". Whenever you navigate to a folder, Bridge will copy the cache from the folder to the main cache and then present thumbnails and previews from the main cache. Copying the cache from the folder to the main is very fast, usually just a few seconds. If you do this, you may be able to leave the main cache on the SSD.
2. Make the cache size huge and leave the "Automatically Export Cache to Folders" off. Now you have all your eggs in one basket. When you navigate to a folder Bridge will look in the main cache only for thumbs and previews. If they are not there, Bridge will generate them.
Bridge has a bad habit of sometimes getting confused and re-generating thumbs/previews even though they already exist. That problem has come and gone and come back again with the various updates. It's annoying becasue the generating of thumbs and previews takes a long time.
Don't make the main cache size huge AND export cache to folders. That's a waste of space because you are duplicating thumbs and previews in 2 places.