Where's my file?

This is strange: the code below works perfectly. But when I look for the file String.txt in sdcard using the tablet's file manager, or using windows explorer, simply can not find the file.

Code:
-------------------
If File.ExternalWritable = False Then
Msgbox("Cannot write on storage card.", "")
Return
End If

File.WriteString(File.DirRootExternal, "String.txt", _
"This is some string" & CRLF & "and this is another one.")

Msgbox(File.ReadString(File.DirRootExternal, "String.txt"), "")

-------------------

I enabled the option to show hidden files and nothing.

The application finds the file but the file manager and windows explorer not.

There is another thing: if I change the file name to one that I have manually copied to the sdcard, it is not found.

I tried the example of the file dialog, and it shows files completely different from the root of the sdcard.