This problem occurs because Macrium Reflect requires elevated Administrator privileges to run. When a network share is mapped to a drive letter in Windows Explorer this is done using standard user account privileges not elevated privileges. Windows makes the mapped share 'visible' only to the standard user not the elevated Administrative user required to run Macrium Reflect.
1. Use the UNC path to access the network folder
Rather than browsing for mapped drive letter you can locate the same folder by entering/browsing for the full Universal Naming Convention (UNC) path, i.e, \\server\share.
In the above example the Server name is 'MyNas' the Share name is 'public' and the folder is 'MyFolder'. So the UNC path is \\MyNas\public\MyFolder'.
2. Alternatively, change Windows to make mapped shares visible
Note: This solution involves changing the Windows registry.
1. Press the Windows button, type 'regedit.exe' and press enter.
2. In the Windows that opens, navigate to:
3. Point to New, and then click DWORD Value.
4. Type EnableLinkedConnections, and then press ENTER.
5. Right-click EnableLinkedConnections, and then click Modify.
6. In the Value data box, type 1, and then click OK.
7. Exit Registry Editor, and then restart the computer.
After making this registry change network shares mapped to a drive letter will be visible to all programs, including those that require elevated privileges.