Harddisk1 => Defined

I have an IBM 9110-51A p Series Server with AIX 5.3.
It has 2 scsi hard disks hdisk0 and hdisk1 (and 4 on disk array).
hdisk1 is is "Defined".
When hdisk1 began "defined" i have a new one unknown disk "available" which called hdisk6.
The left led of the case of hdisk1 on the server is green, the right is off.
When i run



I get "hdisk1 removed".
I cant take a response from hdisk1 from the because it shows me the hdisk1 as missed (M).
I have HMC where i can get more infos .
Can someone help me find and fix the problem so hdisk1 become again available?
What i have to do?
I have a new one IBM scsi hard disk if needed.

Thank you in advance.