A quick google search reveals that Operating system error 112 means "There is not enough space on the disk.". Hence I would guess that there isn't enough space for the backup whereever you're trying to write it.
Free up some space or take the backup somewhere else.
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