Hi all, (forgive my poor English, I'm French

)
I am facing a problem with my NAS. I have a hard drive that contained data (Disk B) which is a Seagate ST2000DL003 and a blank hard disk (Disk A) which is a Western Digital WD20EARS.
So I installed the Western Digital in the NAS (left) and I formatted and then I connected the Seagate on my PC and have transferred files from PC to NAS. Until here everything worked fine.
But when I tried to install the Seagate (right then) in the NAS refused to start, the diodes of the two discs are lit in blue-violet and the power light flashes continuously.
I then tried to reset the NAS, he started it right (blue LED and orange for Seagate WD (Normal I think because it is formatted in NTFS and not EXT3) and I could access the page formatting disks, only when I format the Seagate, nothing happens the formatting stays at 0%
I restart without the Seagate and the NAS will not start, the power light flashes continuously ...
Even diskless in the NAS refuses to start ...
How can I format the Seagate without erasing data from WD?
Thank you that you look at my problem