It looks like something went wrong with your screenshot posting (it isn't in the moderator queue, so something else must be going on).
It should boot up fairly quickly after this - not sure what is going on. Adding a disk does involve a brief test, followed by a resync. The system performance (including the web ui) does suffer some, but it should still boot. Is there anything on the LCD panel?
If it continues, I think you'll need to power down the NAS and pulling the disk drive. After that I suggest testing it in a Windows PC with vendor tools (seatools for Seagate; lifeguard for Western Digital). You can connect the disk to the PC either with SATA or with a USB adapter/dock.
FWIW, we normally recommend hot-inserting new disks (NAS running).
