qBittorrent official forums

I turned on my laptop and then when the client started, qBittorrent started redownloading almost all of my finished torrents that were already seeding.

How can I fix this please? The save destination of the torrents and files is a external hard drive and it's currently attached so I don't know what spawned this.

If it's an external drive, it's likely you removed the drive while qBittorrent was running. Or when the PC started, the external drive was not yet mounted. It can happen with external drives sadly.