Bricked M3800 - need help with BIOS restore if possible

I have a Precision M3800 that has always been reliable and useful for certain tasks. I was having trouble running a graphics-intensive program and eventually used SupportAssist to do some upgrades. That led to trying to update the BIOS from A09 to A14. It would not initiate from within SupportAssist ("Installing" for a very long time), but when I downloaded M3800-A14_x64.exe and ran it manually as Administrator, the update started. There was a dialog box showing green squares as data was written, but it hung at 28% and then eventually BSOD. Then it would not  turn off. Eventually I disconnected A/C and let the battery run down. Then the power key did nothing. Looking around online, I learned to hold END while inserting the power cord. Then it would run the fan at top speed, but black screen. After some more research I tried (on another computer) writing the hdr and rom files for A14 and then for A09, and putting them on a FAT32-formatted USB drive, inserting that into many different USB jacks. Then I read articles about having to change the name of the hdr and rom files and tried several suggested names (eg ZAU50A14.ROM) which also did not work. Does anyone have advice on this?