Yes, that's database mirroring. No, it would not have caused the backup error. It's likely to cause you lots more fun unless you resume the mirroring or remove the mirroring entirely (like full transaction logs, drives out of space and such entertainment)
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability