Using external SSD for After Effects -problems

This sounds like I'm missing something obvious, but as I'm not all that tech-savvy I'll post my question and (maybe) let you have a good laugh.

I've just purchased a 1 TB LaCie SSD external drive, as I want to use that to make my AE work smoother. My AE keeps crashing (crashed 23 times over last weekend) I'm guessing because I'm running out of memory using my iMac hard drive. In the AE Preferences - Media & Disc Cache, I've now chosen a folder on my SSD for Enable Disc Cache and Conformed Media Cache and pressed OK. All looks fine.

However, when I start to edit all cache goes straight to my old cache folders on my iMac, despite those not being a chosen option anymore. Nothing goes into my SSD disc folders, and my crashing problem remains.

What am I missing?