Windows 7, Disk is offline because it has a signature collision
When I switched on today my second hard drive I had the following issue displayed
Running Diskpart as administrator proved that it was the issue
As you can see disk 2 and disk 4 both have the 00024A91 signature hence the collision.
Following the advise here, I changed the signature of the drive decreasing from 1 unit.
Than I brought back the disk online using Disk Management
And everything went back to normal