What is the difference between "last scan processed" and "last processed scan"?

Essentially there's not much difference: in both sentences, the relevant three words refer to the last to be processed of all the scans you've processed.

But if you want to go into minor details and nuances, there's a slight difference in which part is 'stressed':

• "the last scan processed" means the last scan that you processed (focus is on the processing of the scans)
• "the last processed scan" means the last of all the processed scans (focus is on the scans, which have been processed).

There is no difference.

The phrase 'last scan' as opposed to 'last processed scan' could have some ambiguity, because the former could refer to the last of all the scans, not just the most recent.

However in this specific context, with the addition of the word 'processed' and the rest of the sentence, they mean the exact same thing, and can only mean the exact same thing.

In both cases, the meaning is 'the scan that was last to be processed'. English allows the past participle to be used as an adjective, and while our adjectives usually precede the noun, they don't always have to.