diff --git a/tools/upload.py b/tools/upload.py
index 2b7016cc95..29819e5375 100755
--- a/tools/upload.py
+++ b/tools/upload.py
@@ -6,17 +6,17 @@
 # First parameter is pyserial path, second is esptool path, then a series of command arguments
 # i.e. upload.py tools/pyserial tools/esptool write_flash file 0x0
 
-import sys
 import os
+import sys
 import tempfile
 
 sys.argv.pop(0) # Remove executable name
-toolspath = os.path.dirname(os.path.realpath(__file__)).replace('\\', '/') # CWD in UNIX format
+toolspath = os.path.dirname(os.path.realpath(__file__))
 try:
-    sys.path.insert(0, toolspath + "/pyserial") # Add pyserial dir to search path
-    sys.path.insert(0, toolspath + "/esptool") # Add esptool dir to search path
+    sys.path.insert(0, os.path.join(toolspath, "pyserial")) # Add pyserial dir to search path
+    sys.path.insert(0, os.path.join(toolspath, "esptool")) # Add esptool dir to search path
     import esptool # If this fails, we can't continue and will bomb below
-except Exception:
+except ImportError:
     sys.stderr.write("pyserial or esptool directories not found next to this upload.py tool.\n")
     sys.exit(1)
 
@@ -26,7 +26,7 @@
 erase_addr = ''
 erase_len = ''
 
-while len(sys.argv):
+while sys.argv:
     thisarg = sys.argv.pop(0)
 
     # We silently replace the 921kbaud setting with 460k to enable backward
@@ -45,25 +45,30 @@
     elif thisarg == 'write_flash':
         write_addr = sys.argv.pop(0)
         binary = sys.argv.pop(0)
-    elif len(thisarg):
+    elif thisarg:
         cmdline = cmdline + [thisarg]
 
 cmdline = cmdline + ['write_flash']
-if len(write_option):
+if write_option:
     cmdline = cmdline + [write_option]
 cmdline = cmdline + ['--flash_size', 'detect']
 cmdline = cmdline + [write_addr, binary]
 
 erase_file = ''
-if len(erase_addr):
+if erase_addr:
     # Generate temporary empty (0xff) file
     eraser = tempfile.mkstemp()
     erase_file = eraser[1]
-    os.write(eraser[0], bytearray([255] * int(erase_len, 0)))
+    os.write(eraser[0], bytearray([0xff] * int(erase_len, 0)))
     os.close(eraser[0])
-    cmdline = cmdline + [ erase_addr, erase_file ]
+    cmdline = cmdline + [erase_addr, erase_file]
 
-esptool.main(cmdline)
-
-if len(erase_file):
-    os.remove(erase_file)
+try:
+    esptool.main(cmdline)
+except esptool.FatalError as e:
+    sys.stderr.write('\nA fatal esptool.py error occurred: %s' % e)
+finally:
+    if erase_file:
+        os.remove(erase_file)
+    if sys.exc_info:
+        sys.exit(2)