Directory.GetLogicalDrives() does not return network drives when a process is running with administrator privileges. This code returns all logical drives including network drives whether it runs in administrator or not.

DirectoryEx.cs

using System.Collections.Generic;
using System.IO;
using System.Runtime.InteropServices;
using System.Text;

namespace Mpga
{
    /// <summary>
    /// Directory.GetLogicalDrives() does not return network drives when a process 
    /// is running with administrator privileges.
    /// 
    /// This code returns all logical drives including network drives
    /// whether it runs in administrator or not.
    /// </summary>
    public static class DirectoryEx
    {
        [DllImport("mpr.dll", CharSet = CharSet.Unicode, SetLastError = true)]
        public static extern int WNetGetConnection(
            [MarshalAs(UnmanagedType.LPTStr)] string localName,
            [MarshalAs(UnmanagedType.LPTStr)] StringBuilder remoteName,
            ref int length);

        public static string[] GetLogicalDrives()
        {
            List<string> list = new List<string>();
            list.AddRange(Directory.GetLogicalDrives());

            StringBuilder sb = new StringBuilder();
            foreach (var e in list)
            {
                sb.Append(e[0]);
            }
            string driveLetters = sb.ToString().ToUpper();
            for (char c = 'C'; c <= 'Z'; c++)
            {
                if (driveLetters.IndexOf(c) < 0)
                {
                    var uncPath = new StringBuilder(512);
                    var len = uncPath.Capacity;
                    WNetGetConnection($"{c}:", uncPath, ref len);
                    if (uncPath.Length > 0)
                    {
                        list.Add($"{c}:\\");
                    }
                }
            }
            list.Sort();
            return list.ToArray();
        }
    }
}