Keeping old blocks on an external hard drive and latest blocks on an SSD I would like to keep the latest 512 MB of blocks on my SSD (using pruning?) and the rest of the blockchain on an external non-SSD hard-drive. Is there a way to do this already, or do I need to write a script to accomplish this?
Blocks are not accessed under normal operation, except:  When a peer fetches one (and the most recent block is usually served out of an in memory cache), when there is a reorginization that must undo the effect of a block (which is pretty rare), or when you use an RPC to look up a historic block.

With that in mind, perhaps you can just put your blocks directory on the other disk.

If you set your node to be pruned but set the pruned amount much larger than the amount of block data, you will avoid peers fetching large numbers of old blocks from you (but still keep the data around).

If you found high disk accesses while in this configuration, I think thats something that the project would like to fix.