Hi Barty,
I understand that you are logging a lot of data on your cFP-2020 and that you need to make more space for new data. The best option is to transfer data from the cFP-2020 to external memory on a host computer. Compressing data will simply delay the inevitable shortage of memory space. I will look to see if there is a way to compress data on the cFP, but in the mean time, I think transferring data is the best approach. This document should give you a good start if you're new to datalogging with the cFP-2020.
Getting Started with Datalogging and the FieldPoint RT Modules
http://zone.ni.com/devzone/conceptd.nsf/webmain/4312c58b7f45335486256b1a006b2191?OpenDocument
Thanks for contacting NI,
Marcus G.
Applications Engineer
National Instruments