diff --git a/app/logical/storage_manager.rb b/app/logical/storage_manager.rb
index 56f5fc75a..94a957bc4 100644
--- a/app/logical/storage_manager.rb
+++ b/app/logical/storage_manager.rb
@@ -3,22 +3,25 @@
 # are handled by a StorageManager.
 #
 # A StorageManager has methods for saving, deleting, and opening files, and for
-# generates URLs for images.
+# generating URLs for files.
 #
 # @abstract
 # @see StorageManager::Local
+# @see StorageManager::Mirror
+# @see StorageManager::Rclone
 # @see StorageManager::SFTP
 class StorageManager
   class Error < StandardError; end
 
-  attr_reader :base_url, :base_dir
+  attr_reader :base_url
 
   # Initialize a storage manager object.
-  # @param base_url [String] the base URL where images are stored (ex: "https://cdn.donmai.us/")
-  # @param base_dir [String] the base directory where images are stored (ex: "/var/www/danbooru/public/images")
-  def initialize(base_url: nil, base_dir: nil)
-    @base_url = base_url.to_s.chomp("/")
-    @base_dir = base_dir.to_s
+  #
+  # @param base_url [String, nil] the base URL where files are served from (ex:
+  # "https://cdn.donmai.us"), or nil if the files don't have an URL (they're
+  # stored in a publicly inaccessible location).
+  def initialize(base_url: nil)
+    @base_url = base_url
   end
 
   # Store the given file at the given path. If a file already exists at that
@@ -41,16 +44,16 @@ class StorageManager
 
   # Return a readonly copy of the file located at the given path.
   # @param path [String] the remote path of the file to open
-  # @return [MediaFile] the image file
+  # @return [File] the file
   def open(path)
     raise NotImplementedError, "open not implemented"
   end
 
+  # Return the full URL of the file at the given path, or nil if the file
+  # doesn't have an URL.
+  # @return [String, nil] the file URL
   def file_url(path)
+    return nil if base_dir.nil?
     File.join(base_url, path)
   end
-
-  def full_path(path)
-    File.join(base_dir, path)
-  end
 end
diff --git a/app/logical/storage_manager/local.rb b/app/logical/storage_manager/local.rb
index bb68b4fde..c79a990b0 100644
--- a/app/logical/storage_manager/local.rb
+++ b/app/logical/storage_manager/local.rb
@@ -2,6 +2,14 @@
 class StorageManager::Local < StorageManager
   DEFAULT_PERMISSIONS = 0o644
 
+  attr_reader :base_dir
+
+  # @param base_url [String] the base directory where files are stored (ex: "/home/danbooru/public/data")
+  def initialize(base_dir: nil, **options)
+    @base_dir = base_dir.to_s
+    super(**options)
+  end
+
   def store(io, dest_path)
     temp_path = full_path(dest_path) + "-" + SecureRandom.uuid + ".tmp"
 
@@ -24,4 +32,10 @@ class StorageManager::Local < StorageManager
   def open(path)
     File.open(full_path(path), "r", binmode: true)
   end
+
+  protected
+
+  def full_path(path)
+    File.join(base_dir, path)
+  end
 end
diff --git a/app/logical/storage_manager/null.rb b/app/logical/storage_manager/null.rb
index 98cf9d8ef..916bf8252 100644
--- a/app/logical/storage_manager/null.rb
+++ b/app/logical/storage_manager/null.rb
@@ -1,8 +1,8 @@
 # A null StorageManager that doesn't store files at all. Used for testing or
 # disabling backups.
 class StorageManager::Null < StorageManager
-  def initialize(base_url: "/", base_dir: "/")
-    super
+  def initialize
+    super(base_url: nil)
   end
 
   def store(io, path)
diff --git a/app/logical/storage_manager/rclone.rb b/app/logical/storage_manager/rclone.rb
index 9fa9d843b..715a0f542 100644
--- a/app/logical/storage_manager/rclone.rb
+++ b/app/logical/storage_manager/rclone.rb
@@ -5,11 +5,12 @@
 # @see https://rclone.org/
 class StorageManager::Rclone < StorageManager
   class Error < StandardError; end
-  attr_reader :remote, :bucket, :rclone_path, :rclone_options
+  attr_reader :remote, :bucket, :rclone_path, :rclone_options, :base_dir
 
-  def initialize(remote:, bucket:, rclone_path: "rclone", rclone_options: {}, **options)
+  def initialize(remote:, bucket:, base_dir: nil, rclone_path: "rclone", rclone_options: {}, **options)
     @remote = remote
     @bucket = bucket
+    @base_dir = base_dir.to_s
     @rclone_path = rclone_path
     @rclone_options = rclone_options
     super(**options)
@@ -37,4 +38,8 @@ class StorageManager::Rclone < StorageManager
   def key(path)
     ":#{remote}:#{bucket}#{full_path(path)}"
   end
+
+  def full_path(path)
+    File.join(base_dir, path)
+  end
 end
diff --git a/app/logical/storage_manager/sftp.rb b/app/logical/storage_manager/sftp.rb
index 2984be307..bb10aa329 100644
--- a/app/logical/storage_manager/sftp.rb
+++ b/app/logical/storage_manager/sftp.rb
@@ -10,10 +10,11 @@ class StorageManager::SFTP < StorageManager
     non_interactive: true
   }
 
-  attr_reader :hosts, :ssh_options
+  attr_reader :hosts, :ssh_options, :base_dir
 
-  def initialize(*hosts, ssh_options: {}, **options)
+  def initialize(*hosts, base_dir: nil, ssh_options: {}, **options)
     @hosts = hosts
+    @base_dir = base_dir.to_s
     @ssh_options = DEFAULT_SSH_OPTIONS.merge(ssh_options)
     super(**options)
   end
@@ -73,4 +74,8 @@ class StorageManager::SFTP < StorageManager
       end
     end
   end
+
+  def full_path(path)
+    File.join(base_dir, path)
+  end
 end
diff --git a/test/test_helper.rb b/test/test_helper.rb
index 634bfaeb8..65dd62766 100644
--- a/test/test_helper.rb
+++ b/test/test_helper.rb
@@ -51,7 +51,7 @@ class ActiveSupport::TestCase
     @temp_dir = Dir.mktmpdir("danbooru-temp-")
     storage_manager = StorageManager::Local.new(base_url: "https://www.example.com/data", base_dir: @temp_dir)
     Danbooru.config.stubs(:storage_manager).returns(storage_manager)
-    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new(base_url: "/", base_dir: "/"))
+    Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
   end
 
   teardown do