More an O/S question, but I would like to ask here

I have a RasPi and a USB stick plugged into it for logging.
A 128 MG stick. (Yeah: old)

I have fs-ops installed and I run a command daily to indicate the used/available space on the stick.

Today the machine has been rebooted (though it has been done before other times and this not happen) and I was looking at the use. 0. !!??

This is the part of the flow which is used to get the usage:

[{"id":"386cfd91.ae4962","type":"inject","z":"39ea4ac5.b46c1e","name":"Start / test","props":[{"p":"payload","v":"","vt":"date"},{"p":"topic","v":"","vt":"string"}],"repeat":"","crontab":"","once":true,"onceDelay":"2","topic":"","payload":"","payloadType":"date","x":620,"y":4480,"wires":[["e6ba5de5.61847"]]},{"id":"e6ba5de5.61847","type":"Drives","z":"39ea4ac5.b46c1e","name":"","x":570,"y":4560,"wires":[["20499dc9.358b92","e6bea6bd.2aa96"]]},{"id":"20499dc9.358b92","type":"function","z":"39ea4ac5.b46c1e","name":"","func":"var used = msg.payload[8].capacity;\nmsg.payload = used * 100;\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","x":720,"y":4560,"wires":[["2a48360d.1985b2","bdc13e1a.224c","951f3bcc.f09ab8","1a7b1fd6.f0e248"]]}]

(Given there is another input to do this daily.)

You can see the function node extracts [8]'s message for the USB stick.

This had to be modified as it was [9].

Who is changing it form [8] to [9]?

Anyone got a suggestion how I can make the flow track it if it changes where in the array it is?