CFx cards....and Sound Devices CF recorders?

[Philip Perkins]

Can an SD recorder that uses CF cards (the oldies) also use the new CFx cards?  I realize the CFx cards are longer so the slot door on older 6xx machines would have to stay open...

[Display Name]

Unfortunately not. 
 

The old CF cards used ATA interface while the CFExpress uses multiple lanes of PCIe. 
 

The only thing they share are the first two letters. Everything else is totally different. 
 

But you could use a CF to SD adapter card and run the CF card with a SD card inside, instead. 

[Philip Perkins]

True, thanks.