Only look for target disk during Upload (#21804)

Marlin/src/HAL/LPC1768/upload_extra_script.py

@@ -20,6 +20,7 @@ def print_error(e):
 		  'or copy the firmware (.pio/build/%s/firmware.bin) manually to the appropriate disk\n' \
 		  %(e, env.get('PIOENV')))
 
+def before_upload(source, target, env):
 	try:
 		#
 		# Find a disk for upload
@@ -118,3 +119,5 @@ try:
 
 	except Exception as e:
 		print_error(str(e))
+
+env.AddPreAction("upload", before_upload)