On some older Exadatas it can be that Grid Disks belonging to a cell are not found anymore. And not even FORCEing the moutn helps:
FAILGROUP HEADER_STATU COUNT(*) ------------------------------ ------------ ---------- EXA01CEL02 MEMBER 34 EXA01CEL03 MEMBER 34 EXA01CEL04 MEMBER 34 EXA01CEL05 MEMBER 34 EXA01CEL06 MEMBER 34 EXA01CEL07 MEMBER 34 EXA01CEL08 UNKNOWN 34 EXA01CEL09 UNKNOWN 34 EXA01CEL10 UNKNOWN 32 EXA01CEL11 UNKNOWN 34 EXA01CEL12 MEMBER 34 EXA01CEL13 MEMBER 34 EXA01CEL14 MEMBER 34 SQL> alter diskgroup DBFS_DG mount force; alter diskgroup DBFS_DG mount force * ERROR at line 1: ORA-15042: ASM disk "71" is missing from group number "1" ORA-15042: ASM disk "70" is missing from group number "1" ORA-15042: ASM disk "69" is missing from group number "1" ORA-15042: ASM disk "68" is missing from group number "1" ORA-15042: ASM disk "67" is missing from group number "1" ORA-15042: ASM disk "66" is missing from group number "1" ORA-15042: ASM disk "65" is missing from group number "1" ORA-15042: ASM disk "64" is missing from group number "1" ORA-15042: ASM disk "63" is missing from group number "1" ORA-15042: ASM disk "62" is missing from group number "1" ORA-15042: ASM disk "61" is missing from group number "1" ORA-15042: ASM disk "60" is missing from group number "1" ORA-15042: ASM disk "59" is missing from group number "1" ORA-15042: ASM disk "58" is missing from group number "1" ORA-15042: ASM disk "57" is missing from group number "1" ORA-15042: ASM disk "56" is missing from group number "1" ORA-15042: ASM disk "55" is missing from group number "1" ORA-15042: ASM disk "54" is missing from group number "1" ORA-15042: ASM disk "53" is missing from group number "1" ORA-15042: ASM disk "52" is missing from group number "1" ORA-15042: ASM disk "51" is missing from group number "1" ORA-15042: ASM disk "50" is missing from group number "1" ORA-15042: ASM disk "39" is missing from group number "1" ORA-15042: ASM disk "38" is missing from group number "1" ORA-15042: ASM disk "37" is missing from group number "1" ORA-15042: ASM disk "36" is missing from group number "1" ORA-15042: ASM disk "35" is missing from group number "1" ORA-15042: ASM disk "34" is missing from group number "1" ORA-15042: ASM disk "33" is missing from group number "1" ORA-15042: ASM disk "32" is missing from group number "1" ORA-15042: ASM disk "31" is missing from group number "1" ORA-15042: ASM disk "30" is missing from group number "1"
This is fixed with Bug:
16769943 EXADATA: SYSTEM OUTAGE - FOUR CELL NODES IN UNKNOWN STATUS TO ASM