I have a schedule to image my laptop to a large SD card, using the basic full, differential, and incremental settings. The backup works fine until the card fills up, then it fails with 'insufficient disk space', although I've set the 'Run purge before backup' and 'Purge the oldest backup sets if less than nn GB' to delete old backups. It doesn't seem to matter how large I make the minimum free storage for the purge, it doesn't delete anything.

I'm getting tired of canceling a failed backup, going into the Macrium app and deleting a backup manually, then running a new backup manually.

Is there some trick I'm missing?  

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.

Ah, OK - thanks jphugan, I didn't realise it wouldn't delete the current set. There's comfortably room for two full backups, but adding differentials and increments means it can't add a second full backup. I'll play with the retention rules to reduce the numbers.

This is only a secondary backup in case the network goes down and stops the main backup, so it's not critical and I don't mind deleting the current backup to make a new one - having the option, perhaps with a clear warning, would be nice, but I suppose it's better safe than sorry...

I wish Macrium had made it clear that the current backup is never deleted.

Thanks again for the help

The wording does say "previous backup sets" and it's explained in Macrium's excellent online documentation, but in fairness this caught me out too and Macrium Support explained how this worked. If you can't fit a second Full backup after your first one plus all of its Inc/Diffs, set your Retention Policy to 1 Full and have it run the purge before the backup.  The current backup is counted when retention figures are evaluated, so when the next Full is scheduled, all existing backups will be deleted to make room for the next one, with the risk of course that you're deleting all of your backups at that location before new ones are created.  Good luck!