Network mapped drive is same like your HDD drive (but logical mapped on your client machine), why you add '\\' to 'z:' ? '\\' is only used for shared folder not for mapped drive
use below code to copy your files
string updir = @"Z:\Alex\SIB_TEST\abcd.txt";
string destPath = @"D:\Alex_BackUp\Test\abcd.txt";
System.IO.Path.Copy(updir, destPath);
hope it helps