Can't add index to innodb table - "table '#sql_...' is full"

Question

I have a frustrating problem here. I need to add an index to an innodb table which is a few gb in size.

Every time I try this:

alter table x add index y; 

I get

Error Code : 1114
The table '#sql-5f87_30' is full

I already have innodb_data_file_path = ibdata1:10M:autoextend so no limits on the tablespace, and I also have innodb_file_per_table = on.

Also, I've tried changing tmpdir to a dir on a partition with 300 gigs of space.

The server is 64bit Debian, with an ext3 filesystem and 7.5GB ram. There are 4 indexes on the table already.

Can anyone shed light on what the problem could be?

Thanks

Rich