I recently inherited an absolute mess of a network at work, we have a NetVanta 3140 that I have been trying to reset to factory settings. It is currently stuck in bootstrap mode, I have downloaded the default config from here. Using Tera Term and xmodem to send the config to the router, following along with the answer in this post, because I cannot access the one linked with the default config. Whenever I try to boot system flash <primary_image_filename>verify I get an error for "bad biz header." I cannot seem to find anything pertaining to this anywhere. Does anyone have any idea how to fix this?
It sounds like you're confusing the device firmware with its configuration.
The firmware is a large binary file ending in .biz.
The configuration is a smallish text file.
Take care when working with the filesystem not to erase the firmware (except in upgrade situations if you're low on flash storage). To erase configuration, only erase the startup-configuration file.
You'll first need to restore the firmware, then the configuration.
Go here and download the 3140 firmware. It will be a large file ending in .biz. Store this on your computer. If the 3140 is in bootstrap mode you can use TFTP which will be MUCH faster than xmodem. Follow this guide starting on page 23 to load the firmware. Once you've done that you can then use the console to load the default configuration. You can go to enable and then configure from the console with no password.