diff --git a/reactos/drivers/filters/mountmgr/symlink.c b/reactos/drivers/filters/mountmgr/symlink.c
index 80c5b00873e..3fc954dfdbe 100644
--- a/reactos/drivers/filters/mountmgr/symlink.c
+++ b/reactos/drivers/filters/mountmgr/symlink.c
@@ -923,27 +923,35 @@ DeleteSymbolicLinkNameFromMemory(IN PDEVICE_EXTENSION DeviceExtension,
 BOOLEAN
 IsDriveLetter(PUNICODE_STRING SymbolicName)
 {
-    WCHAR Letter;
-    BOOLEAN Result = FALSE;
+    WCHAR Letter, Colon;
 
     /* We must have a precise length */
-    if (SymbolicName->Length != sizeof(DosDevices.Buffer) + 2 * sizeof(WCHAR))
+    if (SymbolicName->Length != DosDevices.Length + 2 * sizeof(WCHAR))
     {
         return FALSE;
     }
 
-    /* Check if len is correct */
-    Letter = SymbolicName->Buffer[sizeof(DosDevices.Buffer) / sizeof(WCHAR)];
-    if (((Letter >= L'A' && Letter <= L'Z') || Letter == (WCHAR)-1) &&
-        SymbolicName->Buffer[(sizeof(DosDevices.Buffer) + sizeof(WCHAR)) / sizeof (WCHAR)] == L':')
+    /* Must start with the DosDevices prefix */
+    if (!RtlPrefixUnicodeString(&DosDevices, SymbolicName, TRUE))
     {
-        /* In case it's not a normal drive letter, check differently */
-        SymbolicName->Length = sizeof(DosDevices.Buffer);
-        Result = RtlEqualUnicodeString(SymbolicName, &DosDevices, TRUE);
-        SymbolicName->Length = sizeof(DosDevices.Buffer) + 2 * sizeof(WCHAR);
+        return FALSE;
     }
 
-    return Result;
+    /* Check if letter is correct */
+    Letter = SymbolicName->Buffer[DosDevices.Length / sizeof(WCHAR)];
+    if ((Letter < L'A' || Letter > L'Z') && Letter != (WCHAR)-1)
+    {
+        return FALSE;
+    }
+
+    /* And finally it must end with a colon */
+    Colon = SymbolicName->Buffer[DosDevices.Length / sizeof(WCHAR) + 1];
+    if (Colon != L':')
+    {
+        return FALSE;
+    }
+
+    return TRUE;
 }
 
 /*