There are some questionable KnowledgeBase articles out there...
The first one linked is using outdated terminology from NI's legacy DAQ driver (which I guess somebody might still find helpful...).
The second one fails to mention that the aiSampleClockTimebase could just be routed through a PFI line rather than having to go through RTSI (EDIT: The STC1's "Master Timebase" can only be overridden via RTSI7 but you shouldn't have to use this).
You can't specify the sample clock timebase using the DAQ Assistant though. When you use the 10 MHz signal on PFI 0 as the external clock through the DAQ Assistant, the hardware will actually try to use this as the sample clock (i.e. sampling off of the 10 MHz signal directly rather than dividing it down to get 5 MHz) which it is too fast for its ADC.
So you'll have to use the lower level DAQmx API. The sample clock timebase source and rate are configured via the following DAQmx Timing properties:
Best Regards,