In trying to rewrite an erroneous EFI for MBP ...

I was unsuccessful; it always had a problem with the S/N as it said (upon attempts writing it) that the size was inaccurate.

I know characters use varying amounts of space due to the hex conversion - is this the problem?

Is this the solution?
- Upon modifying file (S/N) ensure the replacement characters result in the same file size.. ?
- Can I just delete characters if the file size changes?
(I.e., make sure the size of both files match ??)

- PLEASE tell me your favorite OS X hex editor (free ideally ... but doesn't have to be).
- What method can you use to import the bin; I use an arduino via a CH341A. (Not my fav) ... ?
- Please let me know what idiotic things I'm doing - or just

Thank you all very much.

Use Windows and HxD. Then replace your serial on the right side of the window as ASCII text. HxD will put the correct hex values.