Can I kill "repair with keycache"?

I am trying to add an index on a new field that I have added to a MyISAM table that has 8.5 million rows. The request went from "REPAIR BY SORTING" to "REPAIR BY KEYCACHE", which appears to be due to insufficient disk space during sorting. I understand that repairing keycache is not very good and will take a long time - while it runs 1.5 hours.

What happens if I kill this request? Could this damage the table (and the database)? Can I not kill this request and do myisamchk repair?

I have a backup of this table and database, will it save time if I go back and try to add the index again, believing it was caused by disk space?

I would really appreciate any help.