Try this...
CREATE TABLE #tempa
(
idcol INT IDENTITY(1,1),
NAME VARCHAR(100))
INSERT INTO #tempa (NAME)
SELECT 'ServerName1\Dev'
INSERT INTO #tempa (NAME)
SELECT 'SN1\Development'
INSERT INTO #tempa (NAME)
SELECT 'ABCD\1234'
SELECT SUBSTRING(NAME,1,CHARINDEX('\',NAME,1)-1) FROM #tempa