So, the obvious thing is - does 'D:\BEM_Source\Sample' exist in that folder and on the server that the command is being run from? If I run a Bulk Insert from my PC but connected to server 'Fred1', the file has to be on 'Fred1'.

If it is in the correct location, can you open the file with a text editor? Just to make sure it isn't corrupt or has attributes/permissions set that make it impossible to open for some strange reason.