Hi,
Please refer to TeraStation WSS 5000R2 and 5000N2 User Manual.
Page 27 to page 31.
Hard Drive Replacement Examples
A Drive Failure Occurs in Drive 1 or 2
http://d.buffalo.jp/WS5000R2/?dst=AP
There are some Notes. Please read a procedure and notes in advance.
[NOTE]
This is an overview of the replace HDD 1.
1) Shut down the NAS.
2) Replace HDD1.
3) Boot the NAS from HDD2.
4) Repair mirrored volume of the system area by using BUFFALO RAID Builder.
5) Repair other RAID volume.(DATA area).
For more Detail see User Manual.