Try running the following code:

SELECT LTRIM(SUBSTRING (Col1, 1, 20)) AS 'DateTimeStamp',

LTRIM(SUBSTRING(Col1, 71, 6)) AS 'LSN',

LTRIM(SUBSTRING (Col1, 21, 18)) AS 'FileSize',

LTRIM(SUBSTRING (Col1, 40, 1000)) AS 'FileName1'

FROM #OriginalFileList

WHERE TRY_PARSE(LTRIM(SUBSTRING (Col1, 1, 20)) AS DATETIME) IS NULL

ORDER BY LSN

This should list all rows where it cannot convert the first substring to DATETIME.

Drew