Did you encounter any error codes in 3uTools when you flashing an iOS firmware?
Here is the solution about how to fix "Error: Unable to read ECID(Normal)", as below:
Why did this error happen?
A Broken cable
Unstable USB port
Abnormal driver
Partly hardware problem
How to solve the problem?
Change an original cable and try again
Use a rear USB port on your PC
Fix iTunes driver problem in 3uTools
Restore in iTunes
Ask Apple support if the device still in the warranty. (Connect with 3uTools, check the warranty on the "iDevice" page)
Check the hardware problem in the local authoritative repair store