Is Slughorn's Horcrux Memory a Massive Harry Potter Plot Hole?
The vast majority of The Half-Blood Prince plot revolves around Voldemort's past and Professor Slughorn's memories about it… But were they really that important?
Dumbledore goes quite an extra mile to have Horace Slughorn return to Hogwarts and trust Harry.
He tracks his ex-colleague, brings Harry along, and makes an amazing psychological play on Slughorn using him — to ensure Horace's safety and, most importantly, retrieve one certain memory from him, as he later explains to Harry.
Throughout the entire book, as Dumbledore shows his protege other memories regarding Voldemort's past, he constantly stresses how crucial Slughorn's memory is to their cause, and retrieving it is pretty much Harry's homework over and over again.
As we later learn, this memory shows Voldemort's interest in creating Horcruxes.
But by that point we know that Dumbledore has already found and destroyed the Gaunt's ring, one of the Horcruxes, and tracked down another — the Slytherin's Locket.
He clearly knew about the Horcruxes and was actively searching for them, so why was Slughorn's memory even relevant?
This seems like a massive plot hole at first, doesn't it?
However, it's not one.
Sure, Dumbledore knew about the Horcruxes. First of all, he never even believed that Voldemort was gone for good, as Hagrid tells Harry in The Sorcerer's Stone.
Second of all, his suspicions were confirmed when he learned that Quirrell was possessed by the Dark Lord. Third of all, he must have realized everything after he learned about Tom Riddle's diary.
Dumbledore was the greatest wizard of his century, and he definitely knew what kind of magic was able to produce a diary that contains a presumably-dead person's active soul and allows it to take physical shape again.
It's safe to say that Dumbledore knew about Voledmort's Horcruxes since The Chamber of Secrets.
But there's something he didn't know: the number of Horcruxes. Voldemort's soul in Quirrell was allegedly destroyed and the diary obviously contained another fragment.
Dumbledore understood there was more than one Horcrux, but he had no idea just how many there were, and he hoped that Slughorn's memory contained an answer.
This is why it was so important for him to retrieve said memory, and this is not a plot hole. The Headmaster knew about Horcruxes and was on the hunt for them.
He has been locating people who had ties to Voldemort in the past to get their memories to gather hints about the nature of Horcruxes, but he still needed to know the scale.
Slughorn, on the other hand, wasn't aware of just how much Dumbledore knew at that point, and he acted like a coward by withdrawing information. Take a look for yourself.