This will return the string before the last backslash. It'll work if there's more than 2.
DECLARE @STR VARCHAR(200) = 'W:\ABC\F1.bak'
SELECT LEFT(@Str, LEN(@Str) - CHARINDEX('\', REVERSE(@Str)))
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