I have a Intel Turbo Memory 2GB and wonder if it can be used in the E420s? I installed it in the free Mini PCIe socket, but it doesn't "show" anywhere.
Although this empty socket closely resembles a mini PCI-e physically, it is actually a mSATA port and will not recognize a mini PCI-e if inserted.
You can buy a mSATA SSD for that port and use it as boot partition - a commonly used model is the Intel (310 series) X-25-M mSATA. See page 71-72 of the Edge E420s Hardware Maintenance Manual for installation details.