|The Cacti Manual|
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Pay attention to custom scripts. It is required, that external commands called from there are in the $PATH of the cactiuser running the poller. It is therefor recommended to provide /full/path/to/external/command
User "criggie" reported an issue with running smartctl. It was complaining "you are not root" so a quick chmod +s on the script fixed that problem.
Secondly, the script was taking several seconds to run. So cacti was logging a "U" for unparseable in the debug output, and was recording NAN. So my fix there was to make the script run faster - it has to complete in less than one second, and the age of my box made it difficult to accomplish.