While getting a new 1.5TB disk for my Media to be shared on my local network via my AirPort Extreme I discovered a weird problem. I got the disk pre-formatted with an FAT32 partition, since I wanted to store larger files I reformatted it to HFS+ simply by selecting the *partition* and selecting erase.
it showed the “Disk needs repair” message. So I reconnected it to my Mac and checked the disk, which showed no errors, but would not mount on the Airport. After some trial an error I figured the problem to be that there is a difference between formatting the drive and the partition. When looking at the *drive* in Disk Utility it still showed FAT32, but selecting the *partition* it showed HFS+.
The solution to the problem is formatting the *drive* HFS+, afterwards the Airport mounts the disk just fine, and all my media now resides on my network for my devices to see.