I was fighting right now with my favorite Operating System and ASM :-)
I just installed Oracle Grid Infrastructure on a standalone Windows 2008 R2 server (successfully) and wanted to stamp some disks with the tool asmtoolg.exe. After I started the asmtoolg from a command prompt, I receive immediately an error message:
ASM-00203: No valid devices found
Cool :-) Oracle Metalink brings back following result:
After a little bit playing, I found following solution:
Open your Windows Explorer, navigate to your %ORACLE_HOME%\bin and do a right-click on the tool asmtoolg.exe and chose the Option "Run as administrator" and be happy :-) Now I could run the asmtoolg.exe without any error messages.
I was logged in as a local administrator and the above error message came up for me, only after I used above method I could stamp my disks. I really love my favorite Operating System ...