Replace disk in Readynas DUO 1

Hi, last week I had some troubles with disk in my ReadyNas DUO. Disk 1 is original Seagate Barracuda 7200.10, Device Model: ST3750640AS and it is OK (or I hope so). Two and a half year ago I added second disk - Western Digital Caviar Green WDC WD20EARS-00MVWB0 and last week I received message something about Reseting SATA 2 interface, I am not able to find it in the log:

May 23 14:45:58 nas-02-55-E4 kernel: lp_timer_expiry
May 23 14:45:58 nas-02-55-E4 kernel: djo,  chn=1, dj_raid=0, chns=0 source=0 disks=0 parity=0, chn_image=0 sector=679399712/287ed120,sects=32, need_IO=0
May 23 14:45:58 nas-02-55-E4 kernel: djo1,  chn=1, X_mode=NO_RAID, chns=0 source=0 disks=0 parity=0, sects=679399712/287ed120 cmd=0
May 23 14:45:58 nas-02-55-E4 kernel: lp_timer_expiry
May 23 14:45:58 nas-02-55-E4 kernel: 
May 23 14:45:58 nas-02-55-E4 kernel: o_djo,  chn=1, X_raid=0, 0 0 0 0,got_m=0
May 23 14:45:58 nas-02-55-E4 kernel: o_djo1, chn=1, X_raid=0, 0 0 0 0
May 23 14:45:58 nas-02-55-E4 kernel: ==== SATA init channel 1
May 23 14:45:58 nas-02-55-E4 kernel: After INIT SATA channel 1, retry=414, sata=113, status=50
May 23 14:45:58 nas-02-55-E4 kernel: Doing retry on request: 81f57818
May 23 14:45:58 nas-02-55-E4 kernel: Warning:chn 1, there is a pending retry on req 81f57818, current req 81f57818
May 23 14:45:58 nas-02-55-E4 kernel: hwif->irq = 33, retry 1
May 23 14:45:58 nas-02-55-E4 kernel: sata_hotplug: /sbin/hotplug retry hdeUser mode helper start.

In Frontview disk 2 was marked "suspected", and SMART+ button was disabled for this disk drive. I tried to copy some files and after a while I restarted ReadyNas DUO. After restart disk wask marked DEAD, but partition D (disk 2) and all shares was accessible. So I made some backup jobs, and I was able to backup almost all my data from "dead" disk to USB drive.
I bought new 2T disk - WD20EFRX and tried to replace it. So I removed old disk from tray 2, replaced drive with new one and put it back into DUO (system was ON all the time).
But there was a problem - I did not see new disk in Frontview, there was stil old one marked "DEAD". So I tried to reboot NAS with no effect. Then I opened Frontview and in Volume Settings I deleted volume D (DEAD disk). After a while I received an error - Deleting volume failed. Volume D disapeared as well as all shares on dead disk. but in health status there was still old dead disk. So I rebooted NAS once again and in RAIDar I saw file system check performed to 100% but then NAS was no longer visible in RAIDar and blue LED was flashing. So I tried to reboot it few more times (after many hours waiting), no effect. So i searched forum, removed new disk - nothing. Then I performed OS-Reload (pressed Reset while powering on, and after first LED blink released Reset) and NAS was back online, seeing all data (on undamaged disk).
So my problem is - how should I put new disk into NAS? I havent mentioned that I am not sure what RAID mode I am using. I know that there is no mirror or striping, but I am not sure whether it is RAID, XRAID or Volume settings. How could I find it out?
Is there any possibility to insert new disk and create new volume without factory reset? I want my first disk left untouched.

Thanks