Hi JB,
I have looked into this issue. I haven't been able to solve this issue yet as it seem like a BUG in File adapter (Not sure so far). But i can explain what’s going wrong.
Biztalk does create a Temp file Named: "[Your complete file name].BTS-WIP"
When it finishes writing the temp file to the specified location, it tries to rename the file to "[Your complete file name]". But unfortunately the file is already open by the existing stream and Biztalk gets the "Sharing Violation" error. Upon getting the error it deletes the "[Your complete file name].BTS-WIP" file.
You can test this your self as well. Just remove the delete permission from the Folder where Biztalk tries to create the File. Run your process and see that the "[Your complete file name].BTS-WIP" is still there as the Biztalk process was not given permission to delete the file.
I have been trying to resolve this issue but no luck so far. But I thought to share my findings with you, so you can look into this as well.
Look through these:
File Lock or Access Denied Error Message When You Save Files Over the Network
http://support.microsoft.com/default.aspx?scid=kb;EN-US;812937
Files on Network Shares Open Slowly or Read-Only or You Receive an Error Message
http://support.microsoft.com/?kbid=814112
turn off multi user editing
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q292122
Program Opens a File As Read-Only or Returns a Sharing Violation
http://support.microsoft.com/default.aspx?scid=kb;EN-US;328170
Hopefully there will be any other solution without a need to create a custom adapter. Keep the post updated if you find any thing.
Cheers
Zia