diff --git a/reactos/drivers/filters/mountmgr/mntmgr.h b/reactos/drivers/filters/mountmgr/mntmgr.h
index d0cf8a96151..e51188ae4cb 100644
--- a/reactos/drivers/filters/mountmgr/mntmgr.h
+++ b/reactos/drivers/filters/mountmgr/mntmgr.h
@@ -13,7 +13,7 @@
 #undef IsEqualGUID
 #endif
 
-#define IsEqualGUID(rguid1, rguid2) (!RtlCompareMemory(rguid1, rguid2, sizeof(GUID)))
+#define IsEqualGUID(rguid1, rguid2) (RtlCompareMemory(rguid1, rguid2, sizeof(GUID)) == sizeof(GUID))
 
 #define FILE_READ_PROPERTIES  0x00000008
 #define FILE_WRITE_PROPERTIES 0x00000010
diff --git a/reactos/drivers/filters/mountmgr/mountmgr.c b/reactos/drivers/filters/mountmgr/mountmgr.c
index cddbcc446f5..f623976a74e 100644
--- a/reactos/drivers/filters/mountmgr/mountmgr.c
+++ b/reactos/drivers/filters/mountmgr/mountmgr.c
@@ -132,8 +132,8 @@ CreateNewDriveLetterName(OUT PUNICODE_STRING DriveLetter,
     NTSTATUS Status = STATUS_UNSUCCESSFUL;
 
     /* Allocate a big enough buffer to contain the symbolic link */
-    DriveLetter->MaximumLength = sizeof(DosDevices.Buffer) + 3 * sizeof(WCHAR);
-    DriveLetter->Buffer = AllocatePool(sizeof(DosDevices.Buffer) + 3 * sizeof(WCHAR));
+    DriveLetter->MaximumLength = DosDevices.Length + 3 * sizeof(WCHAR);
+    DriveLetter->Buffer = AllocatePool(DriveLetter->MaximumLength);
     if (!DriveLetter->Buffer)
     {
         return STATUS_INSUFFICIENT_RESOURCES;
@@ -143,9 +143,9 @@ CreateNewDriveLetterName(OUT PUNICODE_STRING DriveLetter,
     RtlCopyUnicodeString(DriveLetter, &DosDevices);
 
     /* Update string to reflect real contents */
-    DriveLetter->Length = sizeof(DosDevices.Buffer) + 2 * sizeof(WCHAR);
-    DriveLetter->Buffer[(sizeof(DosDevices.Buffer) + 2 * sizeof(WCHAR)) / sizeof (WCHAR)] = UNICODE_NULL;
-    DriveLetter->Buffer[(sizeof(DosDevices.Buffer) + sizeof(WCHAR)) / sizeof (WCHAR)] = L':';
+    DriveLetter->Length = DosDevices.Length + 2 * sizeof(WCHAR);
+    DriveLetter->Buffer[DosDevices.Length / sizeof(WCHAR) + 2] = UNICODE_NULL;
+    DriveLetter->Buffer[DosDevices.Length / sizeof(WCHAR) + 1] = L':';
 
     /* If caller wants a no drive entry */
     if (Letter == (UCHAR)-1)
@@ -158,7 +158,7 @@ CreateNewDriveLetterName(OUT PUNICODE_STRING DriveLetter,
     else if (Letter)
     {
         /* Use the letter given by the caller */
-        DriveLetter->Buffer[sizeof(DosDevices.Buffer) / sizeof(WCHAR)] = (WCHAR)Letter;
+        DriveLetter->Buffer[DosDevices.Length / sizeof(WCHAR)] = (WCHAR)Letter;
         Status = GlobalCreateSymbolicLink(DriveLetter, DeviceName);
         if (NT_SUCCESS(Status))
         {
@@ -187,16 +187,18 @@ CreateNewDriveLetterName(OUT PUNICODE_STRING DriveLetter,
     /* Try to affect a letter (up to Z, ofc) until it's possible */
     for (; Letter <= 'Z'; Letter++)
     {
-        DriveLetter->Buffer[sizeof(DosDevices.Buffer) / sizeof(WCHAR)] = (WCHAR)Letter;
+        DriveLetter->Buffer[DosDevices.Length / sizeof(WCHAR)] = (WCHAR)Letter;
         Status = GlobalCreateSymbolicLink(DriveLetter, DeviceName);
         if (NT_SUCCESS(Status))
         {
+            DPRINT("Assigned drive %c: to %wZ\n", Letter, DeviceName);
             return Status;
         }
     }
 
     /* We failed to allocate a letter */
     FreePool(DriveLetter->Buffer);
+    DPRINT("Failed to create a drive letter for %wZ\n", DeviceName);
     return Status;
 }
 
@@ -558,7 +560,7 @@ QueryDeviceInformation(IN PUNICODE_STRING SymbolicName,
 
             /* Query unique ID */
             KeInitializeEvent(&Event, NotificationEvent, FALSE);
-            Irp = IoBuildDeviceIoControlRequest(IOCTL_MOUNTDEV_QUERY_DEVICE_NAME,
+            Irp = IoBuildDeviceIoControlRequest(IOCTL_MOUNTDEV_QUERY_UNIQUE_ID,
                                                 DeviceObject,
                                                 NULL,
                                                 0,
@@ -1070,7 +1072,7 @@ MountMgrMountedDeviceArrival(IN PDEVICE_EXTENSION DeviceExtension,
     {
         CurrentDevice = CONTAINING_RECORD(NextEntry, DEVICE_INFORMATION, DeviceListEntry);
 
-        if (RtlEqualUnicodeString(&(DeviceInformation->DeviceName), &TargetDeviceName, TRUE))
+        if (RtlEqualUnicodeString(&(CurrentDevice->DeviceName), &TargetDeviceName, TRUE))
         {
             break;
         }
@@ -1876,7 +1878,7 @@ DriverEntry(IN PDRIVER_OBJECT DriverObject,
                                             &MountedDevicesGuid,
                                             DriverObject,
                                             MountMgrMountedDeviceNotification,
-                                            DeviceObject,
+                                            DeviceExtension,
                                             &(DeviceExtension->NotificationEntry));
 
     if (!NT_SUCCESS(Status))