Just intro the drive and a popup will ask if you want to unlock the spi
Cut the trace - click yes - probe the point - unlocked done. Solder the trace back together - continue with the flashing as usual.
This is what a trace cut should look like when done correctly and professionally. Traces cleaned with fiberglass scratch brush, trace cut then both side fluxed and tinned.
Note: the pic isn't the spot you need to do on the DG-16D4S board - its shown as an example