Problem creating index

Hi to all ....

On my Postgresql 9.1 instance I had a problem with an index.
Using index I get less tuples than expected.
I try to remove index and the query works fine but obviosly the query is
slow so I try to recreate the index.

I run the create index statement but after a lot of time I get this error
message:

ERROR: unexpected end of tape

I try to look in postgresql and syslog log files but I find nothing.

Any suggestion?

Create index statement that I use is:

CREATE INDEX dati_impianto_id_tipo_dato_id_data_misurazione_idx
ON dati
USING btree
(impianto_id , tipo_dato_id , data_misurazione DESC);