Problem access logical drivers in windows services

Hi, I have a problem with access to logical drivers in windows services.
I done "subst f: c:\" for example. I try to search a file with FileInfo.Exists and I'm not found that file and really exists.

Then I tried obtain logical driver with DriveInfo.GetDrives [http://technet.microsoft.com/es-es/l...VS.85%29.aspx]

but I not returned logical drivers.

Any idea?