diff --git a/sys/apis/util.lua b/sys/apis/util.lua
index 771bcbb..2a3fb64 100644
--- a/sys/apis/util.lua
+++ b/sys/apis/util.lua
@@ -414,6 +414,16 @@ end
 
 function Util.writeFile(fname, data)
 	if not fname or not data then error('Util.writeFile: invalid parameters', 2) end
+
+	if fs.exists(fname) then
+		local diff = #data - fs.getSize(fname)
+		if diff > 0 then
+			if fs.getFreeSpace(fs.getDir(fname)) < diff then
+				error('Insufficient disk space for ' .. fname)
+			end
+		end
+	end
+
 	local file = io.open(fname, "w")
 	if not file then
 		error('Unable to open ' .. fname, 2)
diff --git a/sys/apps/cedit.lua b/sys/apps/cedit.lua
index 8032688..5abcee0 100644
--- a/sys/apps/cedit.lua
+++ b/sys/apps/cedit.lua
@@ -13,6 +13,7 @@ if not _G.http.websocket then
 end
 
 if not _G.cloud_catcher then
+  print('Visit https://cloud-catcher.squiddev.cc')
   print('Paste key: ')
   local key = read()
   if #key == 0 then
diff --git a/sys/apps/cshell.lua b/sys/apps/cshell.lua
index 7bc6af6..91b1eab 100644
--- a/sys/apps/cshell.lua
+++ b/sys/apps/cshell.lua
@@ -6,6 +6,7 @@ if not _G.http.websocket then
 end
 
 if not _G.cloud_catcher then
+  print('Visit https://cloud-catcher.squiddev.cc')
   print('Paste key: ')
   local key = read()
   if #key == 0 then
diff --git a/sys/extensions/2.vfs.lua b/sys/extensions/2.vfs.lua
index 3558c60..d80474c 100644
--- a/sys/extensions/2.vfs.lua
+++ b/sys/extensions/2.vfs.lua
@@ -113,6 +113,7 @@ local function splitpath(path)
 end
 
 local function getNode(dir)
+	if not dir then error('Invalid directory', 2) end
 	local cd = fs.combine(dir, '')
 	local parts = splitpath(cd)
 	local node = fs.nodes
@@ -176,6 +177,8 @@ function fs.listEx(dir)
 end
 
 function fs.copy(s, t)
+	if not s then error('copy: bad argument #1') end
+	if not t then error('copy: bad argument #2') end
 	local sp = getNode(s)
 	local tp = getNode(t)
 	if sp == tp and sp.fs.copy then
diff --git a/sys/help/CloudCatcher b/sys/help/CloudCatcher
new file mode 100644
index 0000000..e50dd86
--- /dev/null
+++ b/sys/help/CloudCatcher
@@ -0,0 +1,5 @@
+Cloud Catcher is a web terminal for ComputerCraft, allowing you to interact with any in-game computer in the browser, as well as edit files remotely!
+
+To get started, visit https://cloud-catcher.squiddev.cc/cloud.lua for a session key.
+
+Within Files, press 'c' to edit a file using Cloud Catcher.