I have an older ER-3 which was well beyond the warrenty period and which would not boot. Checking via the console seemed to indicate a corrupted flash drive. This thread motivated me to open the case and take out the flash drive with an initial intention of replacing the drive.
However, since my home workstation is a linux system, I decided to use the standard file system tools to try and correct the file system problem. The flash drive is acually quite small (3+ gb) with 2gb set aside in an unallocated partition. In any case, i ran ext3 file system checks and then repartioned the flash drive, checked the fat boot partition, etc. It appeared to be working correctly on my workstation.
Back into the ER-3, and following the "last resort" instructions (https://help.ubnt.com/hc/en-us/articles/204959514?input_string=edgerouter+lite+not+workin) I reinstalled the OS and have a functioning ER-3 once again.
Too soon to tell if this will last. I had to use a console-usb cable and the wonderful screen application on linux to work at the boot CLI to accomplish the reimage and reinstall.
Brad