What firmware are you using and what addons are you trying to install?
I ask because the standard volume labels for the disks are Volume_1 and Volume_2 not HD-a2 or HD-b2. The latter name sound as if you are using Fun-plug or Alt-F.
In the web UI under 'Management' --> 'Status' --> 'Hard Drive info' you will find the information about your drives. Note the right hand slot is considered to be the primary slot and would normally get the Volume_1 label in a two drive setup. I am thinking your single drive was in the left hand slot and when you you put the new drive in the right hand slot and formatted it primary and secondary drives got changed hence your problem.
Without more information I can't start to help you resolve this problem.
Fireware 1.05. I think that,add-ons are not the reason cause this problem . Because I tried several add-ons , most of them caused webui no response.
They were Koken,Samba,Logitech Media Server,Mediatomb.
I also found some interesting results. When I putted the new dirve in the right slot, some add-ons work fine(eg:Wordpress,Joomla), some are not(aria2,telnetd,sshd...).
You are right, I put the first drive in the left. Now the new putted drive is in the right, but labeled Volume_2.
When I called them HD_a2 and HD_b2, I was in the WINSCP, logined as root user. The path was "/mnt/HD/HD_a2 HD_b2".
I think , the system was designed to locate HD_a2 to be the primary drive. It can be expressed why the single drive in the left slot named HD_a2 and when the newer drive putted in the right it was named HD_a2. But the "Volume_1" was not changed, the newer drive is "Volume_2".
I am thinking, if I exchange the two dirves what will happen. Will it fix the problem ?