The disk is offline because it has a signature collision with another disk that is online
you could try this solution (which I found on the net):
1. Open Command Prompt.
2. type DISKPART - a new window will open.
3. enter command : LIST DISK. This will show you all the connected disk with the offline one.
4. Now select the offline disk using the next command SELECT DISK <disk_no> wheredisk_no is the offline disk number which you can get from step 3 result.
5. Now you have selected the offline disk. Now enter command ONLINE DISK.
1. Open Command Prompt.
2. type DISKPART - a new window will open.
3. enter command : LIST DISK. This will show you all the connected disk with the offline one.
4. Now select the offline disk using the next command SELECT DISK <disk_no> wheredisk_no is the offline disk number which you can get from step 3 result.
5. Now you have selected the offline disk. Now enter command ONLINE DISK.
