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By Hexum064
#50405 I struggled for an entire day with a similar situation. I could not get NodeMCU firmware to flash to my ESP-01. It would never get past 16%. Sometimes it wouldn't even start.

It turned out to be the USB to Serial adapter. I was using some of the really cheep FTDI ones (the ones with the red solder mask). I tried 3 different ones, all with the same result.

Finally ordered an adapter with an Si Labs chip and, bingo! Uploaded just fine. I have still had some failures but usually can get the firmware uploaded after a handful of attempts.

Hope this helps.