Index: /trunk/src/VBox/Main/src-server/linux/USBProxyServiceLinux.cpp
===================================================================
--- /trunk/src/VBox/Main/src-server/linux/USBProxyServiceLinux.cpp	(revision 36996)
+++ /trunk/src/VBox/Main/src-server/linux/USBProxyServiceLinux.cpp	(revision 36997)
@@ -141,4 +141,5 @@
                     pcszUsbFromEnv));
             fValidVBoxUSB = false;
+            pcszUsbFromEnv = NULL;
         }
         if (!fValidVBoxUSB && pcszUsbRoot)
@@ -160,4 +161,6 @@
         }
     }
+    else if (!USBProxyLinuxCheckDeviceRoot(pcszUsbRoot, fSysfsChosen))
+        pcszUsbRoot = NULL;
     if (pcszUsbRoot)
     {
@@ -176,5 +179,6 @@
     }
     else
-        mLastError =   RTDirExists("/dev/vboxusb") ? VERR_VUSB_USB_DEVICE_PERMISSION
+        mLastError =   pcszUsbFromEnv ? VERR_NOT_FOUND
+                     : RTDirExists("/dev/vboxusb") ? VERR_VUSB_USB_DEVICE_PERMISSION
                      : RTFileExists("/proc/bus/usb/devices") ? VERR_VUSB_USBFS_PERMISSION
                      : VERR_NOT_FOUND;
Index: /trunk/src/VBox/Main/testcase/tstUSBProxyLinux.cpp
===================================================================
--- /trunk/src/VBox/Main/testcase/tstUSBProxyLinux.cpp	(revision 36996)
+++ /trunk/src/VBox/Main/testcase/tstUSBProxyLinux.cpp	(revision 36997)
@@ -87,22 +87,18 @@
 } s_testEnvironment[] =
 {
-    /* "sysfs" and root in the environment */
-    { "sysfs", "/dev/bus/usb", NULL, false, NULL, false, VINF_SUCCESS, "/dev/bus/usb", false, VINF_SUCCESS },
-    /* "sysfs" and root in the environment, method-specific init failed */
-    { "sysfs", "/dev/bus/usb", NULL, false, NULL, false, VERR_NO_MEMORY, "/dev/bus/usb", false, VERR_NO_MEMORY },
-    /* "sysfs" and bad root in the environment (should succeed as we don't do checks if the user specifies everything) */
-    { "sysfs", "/dev/bus/usb", "/dev/usbvbox", false, "/proc/usb/bus", false, VINF_SUCCESS, "/dev/bus/usb", false, VINF_SUCCESS },
-    /* "sysfs" and bad root in the environment, method-specific init failed */
-    { "sysfs", "/dev/bus/usb", "/dev/usbvbox", false, "/proc/usb/bus", false, VERR_NO_MEMORY, "/dev/bus/usb", false, VERR_NO_MEMORY },
+    /* "sysfs" and valid root in the environment */
+    { "sysfs", "/dev/bus/usb", "/dev/bus/usb", true, NULL, false, VINF_SUCCESS, "/dev/bus/usb", false, VINF_SUCCESS },
+    /* "sysfs" and valid root in the environment, method-specific init failed */
+    { "sysfs", "/dev/bus/usb",  "/dev/bus/usb", true, NULL, false, VERR_NO_MEMORY, "/dev/bus/usb", false, VERR_NO_MEMORY },
+    /* "sysfs" and bad root in the environment */
+    { "sysfs", "/dev/bus/usb", "/dev/vboxusb", false, "/proc/usb/bus", false, VINF_SUCCESS, "", true, VERR_NOT_FOUND },
     /* "sysfs" and no root in the environment */
     { "sysfs", NULL, "/dev/vboxusb", true, NULL, false, VINF_SUCCESS, "/dev/vboxusb", false, VINF_SUCCESS },
-    /* "usbfs" and root in the environment */
-    { "usbfs", "/dev/bus/usb", NULL, false, NULL, false, VINF_SUCCESS, "/dev/bus/usb", true, VINF_SUCCESS },
-    /* "usbfs" and root in the environment, method-specific init failed */
-    { "usbfs", "/dev/bus/usb", NULL, false, NULL, false, VERR_NO_MEMORY, "/dev/bus/usb", true, VERR_NO_MEMORY },
-    /* "usbfs" and bad root in the environment (should succeed as we don't do checks if the user specifies everything) */
-    { "usbfs", "/dev/bus/usb", "/dev/usbvbox", false, "/proc/usb/bus", false, VINF_SUCCESS, "/dev/bus/usb", true, VINF_SUCCESS },
-    /* "usbfs" and bad root in the environment, method-specific init failed */
-    { "usbfs", "/dev/bus/usb", "/dev/usbvbox", false, "/proc/usb/bus", false, VERR_NO_MEMORY, "/dev/bus/usb", true, VERR_NO_MEMORY },
+    /* "usbfs" and valid root in the environment */
+    { "usbfs", "/dev/bus/usb", NULL, false, "/dev/bus/usb", true, VINF_SUCCESS, "/dev/bus/usb", true, VINF_SUCCESS },
+    /* "usbfs" and valid root in the environment, method-specific init failed */
+    { "usbfs", "/dev/bus/usb", NULL, false, "/dev/bus/usb", true, VERR_NO_MEMORY, "/dev/bus/usb", true, VERR_NO_MEMORY },
+    /* "usbfs" and bad root in the environment */
+    { "usbfs", "/dev/bus/usb", "/dev/vboxusb", false, "/proc/usb/bus", false, VINF_SUCCESS, "", true, VERR_NOT_FOUND },
     /* "usbfs" and no root in the environment */
     { "usbfs", NULL, NULL, false, "/proc/bus/usb", true, VINF_SUCCESS, "/proc/bus/usb", true, VINF_SUCCESS },