diff --git a/chrome/content/zotero-platform/mac/overlay.css b/chrome/content/zotero-platform/mac/overlay.css
index 6ae867d01..fafe83e69 100644
--- a/chrome/content/zotero-platform/mac/overlay.css
+++ b/chrome/content/zotero-platform/mac/overlay.css
@@ -57,6 +57,11 @@
   -moz-padding-start: 8px;
 }
 
+#zotero-tb-sync-warning[error=true]
+{
+	margin-bottom: 2px;
+}
+
 .zotero-tb-button[open="true"] > .toolbarbutton-icon,
 .zotero-tb-button:hover:active > .toolbarbutton-icon, 
 #zotero-pane-stack[fullscreenmode="true"] #zotero-tb-fullscreen > .toolbarbutton-icon {
diff --git a/chrome/content/zotero/xpcom/storage.js b/chrome/content/zotero/xpcom/storage.js
index c3fd037b6..e74caae33 100644
--- a/chrome/content/zotero/xpcom/storage.js
+++ b/chrome/content/zotero/xpcom/storage.js
@@ -888,8 +888,7 @@ Zotero.Sync.Storage = new function () {
 			destFile.QueryInterface(Components.interfaces.nsILocalFile);
 			destFile.setRelativeDescriptor(parentDir, fileName);
 			if (destFile.exists()) {
-				var msg = "ZIP entry '" + fileName + "' "
-					+ " already exists";
+				var msg = "ZIP entry '" + fileName + "' " + "already exists";
 				Zotero.debug(msg, 2);
 				Components.utils.reportError(msg + " in " + funcName);
 				continue;
@@ -984,7 +983,22 @@ Zotero.Sync.Storage = new function () {
 			
 			if (file.isFile()) {
 				Zotero.debug("Deleting existing file " + file.leafName);
-				file.remove(false);
+				try {
+					file.remove(false);
+				}
+				catch (e) {
+					if (e.name == 'NS_ERROR_FILE_ACCESS_DENIED') {
+						Zotero.debug(e);
+						
+						// TODO: localize
+						var msg = "The file '" + file.leafName + "' is in use and cannot "
+							+ "be updated. Please close the file or restart your computer "
+							+ "and try syncing again.";
+						throw (msg);
+					}
+					
+					throw (e);
+				}
 			}
 			else if (file.isDirectory()) {
 				Zotero.debug("Deleting existing directory " + file.leafName);
diff --git a/chrome/content/zotero/xpcom/storage/zfs.js b/chrome/content/zotero/xpcom/storage/zfs.js
index 3ef901319..4a2bc951f 100644
--- a/chrome/content/zotero/xpcom/storage/zfs.js
+++ b/chrome/content/zotero/xpcom/storage/zfs.js
@@ -190,6 +190,11 @@ Zotero.Sync.Storage.Session.ZFS.prototype.downloadFile = function (request) {
 			else {
 				destFile.append(item.key + '.tmp');
 			}
+			
+			if (destFile.exists()) {
+				destFile.remove(false);
+			}
+			
 			// saveURI() below appears not to create empty files for Content-Length: 0,
 			// so we create one here just in case
 			destFile.create(Components.interfaces.nsIFile.NORMAL_FILE_TYPE, 0644);
@@ -252,8 +257,7 @@ Zotero.Sync.Storage.Session.ZFS.prototype.downloadFile = function (request) {
 			var wbp = Components
 				.classes["@mozilla.org/embedding/browser/nsWebBrowserPersist;1"]
 				.createInstance(nsIWBP);
-			wbp.persistFlags = nsIWBP.PERSIST_FLAGS_BYPASS_CACHE
-								| nsIWBP.PERSIST_FLAGS_REPLACE_EXISTING_FILES;
+			wbp.persistFlags = nsIWBP.PERSIST_FLAGS_BYPASS_CACHE;
 			wbp.progressListener = listener;
 			wbp.saveURI(uri, null, null, null, null, destFile);
 		}
diff --git a/chrome/skin/default/zotero/overlay.css b/chrome/skin/default/zotero/overlay.css
index 1c29370a0..58fba9cf5 100644
--- a/chrome/skin/default/zotero/overlay.css
+++ b/chrome/skin/default/zotero/overlay.css
@@ -221,7 +221,6 @@
 #zotero-tb-sync-warning[error=true]
 {
 	list-style-image: url(chrome://zotero/skin/exclamation.png);
-	margin-bottom: 2px;
 }
 
 #zotero-tb-sync {