If you already had a Full at your backup destination, chose to run a Differential, and Reflect created a Full anyway, then it was because it couldn't build a Differential off of the existing Full. You'll see a note about this in the log from that job. If you're using Windows 10, the most likely cause of this behavior is described in this thread
, and just fyi, Microsoft now intends to release new Windows 10 builds every 6 months, with initial rollouts beginning every March and September. That doesn't mean you'll see this behavior with every Windows 10 upgrade since it's possible they won't all require resizing the Recovery partition, but it's certainly a possibility. If that's not what happened in your case, the rule is that Reflect will only append to an existing backup set if it considers the set to be "matching", and for image jobs that means that the existing set must contain exactly
the same partitions as those in the currently running job. So if you for example change the size of a partition that's included in your backup, or add/remove partitions selected for backup in your job, the set will no longer be considered matching, and consequently Reflect will have to create a new Full.
In terms of what to do at this point, you're stuck with the new Full, but also be aware that by default (and as mentioned in that linked thread), your retention policy does NOT apply to non-matching backup sets even when they're in the same folder. This means that your old Full(s) and Diffs will NEVER be purged automatically going forward. If you wish to change that behavior, you can change your retention policy matching setting to "All backups in the destination folder" rather than "Matching backups in the destination folder", but in that case make sure that your destination folder only contains backups generated by this specific backup job, otherwise changing that setting could cause Reflect to start deleting completely unrelated backups. If you don't want to dedicate a folder just to backups generated by this job, you'd have to manually purge your old backups whenever you feel you can do without them.