Turns out we know a bit more about why yesterday's Burning Crusade pre-patch maintenance took so long, as World of Warcraft's community manager pointed to player's in-game mailboxes as the culprit.
Yesterday's pre-patch took a fair bit longer than was expected, as Blizzard finally pushed the pre-patch live early this morning. However, the reason for the delays was given by a community manager last night via the Blizzard forums to explain what exactly was causing the issues.
Apparently, players inboxes was causing a problem with the pre-patch, forcing Blizzard to restore the database where that information is contained. Via Kaivax on the WoW Classic forums:
"There are jobs that copy data and jobs that check the integrity of data and there’s an immense amount of data to process before we even begin the steps that we take to check everything manually. It’s not possible for us to lose anything, but it is possible for us to have an issue that requires a step to be repeated. In today’s case, the step we had to start over was a big one. While performing data integrity checks on the Burning Crusade Classic and Classic Era realms, we discovered an unfortunate issue with all player mail. The nature of the issue necessitated that we restore the portion of the database that contains that information in order to ensure nothing is lost. We’ve been in progress on this restoration for quite some time, but as you can imagine, it’s a huge amount of information."
As far as why the mailboxes themselves caused a problem isn't exactly stated in the statement other than the need to ensure that player's mailboxes remained intact when downtime was over. However, it's not hard to glean as to why mailboxes could be an issue, especially with many players using mailboxes as extensions of their vault space. The comments flowed in with players joking about this on the WoWHead forums, with one player worried about their 500 stacks of Runecloth in the mail, while others telling Blizzard to take as long as they need so as to not lose their valuable mail items.
WoW Classic's pre-patch finally hit this morning, bringing the Burning Crusade era to Classic. Those players looking to get a jump start on the expansion when it officially hits on June 1st can start doing so, leveling their toons to the cap, or even creating a new Blood Elf or Draenei, depending on their allegiances.
Pre-Patch also signals a break for some Classic players, as the choice to remain on the WoW Classic servers or move on to Burning Crusade Classic servers is one being presented to players now. Thankfully, you can copy your player and get the best of both versions, though it will cost you. However, following player feedback, Blizzard lowered the cost of the service from $35 to $15 last week.