Most Valuable Professional
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The purge option only purges old backup SETS, i.e. previous Fulls and any child Inc/Diffs they have. If you only have one Full on the SD card, you only have one set, and since the current set is never considered for purging, that option would do nothing. In that case, you'd need to specify a retention policy that's more feasible with the amount of storage you have, since retention policies can clean house within the current set. That low disk space purge is also only triggered as needed, including in the middle of a running backup; the before/after checkbox affects when the RETENTION RULES (which ARE applied within the current backup set) are applied. Retention policies are by far the preferred method of storage management anyway since as you might have just gathered, the purge option that acts on entire sets can purge a LOT of backup history in one fell swoop depending on your strategy, so that's sort of a last resort measure.
If that doesn't make things clear enough, it would help if you could post your retention settings as well as a screenshot showing which backups currently exist at your destination. And if you're wondering why there isn't a way to run more granular retention policies on demand, it's safer to let new backups fail than delete old ones since after all, a new backup can be run again if needed, but once an old backup is gone, it's basically gone unless you want to deal with trying to do deleted file data recovery. And in some cases, an unintended purge of old backups might create legal/regulatory problems.