Can't write to a network drive

I am using the log4net's FileAppender. It is perfectly fine when the logfile
is set to a directory in the local hard drive. However if the logfile is in
a mapped network drive, log4net refuses to log anything and there is no any
error message either... Did I miss something or it is normal? 

Here is the config for lcoal drive d:\
<param name="File" value="d:\\logfiles\\Log.txt" /> it is working fine.

here is config for a mapped network drive U:\ <param name="File"
value="U:\\logfiles\\Log.txt" /> which doesn't work.

Thanks



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