I think I got you on this one, and I registered an account and made this my first post just because I had the same problem and really wanted answers.
I just SBF'd today after bricking my phone with Team Black Hats zip flashes. I know that it seems super convenient and whatever, but things downloading over my phones wifi with all the things I run sometimes I feel they get interrupted.. And that one missing billionth of a zillionth of a bite throws off the whole process.. So I try to download things over the computer as much as I can I feel its more reliable connection wise.
Anyways, I used team black hats .zip to drop back to unrooted .340 as I was told I should do in order to run team black hats GingerBreadRoot .zip after that. Well after all was said and done, when I installed gingerbread from bootloader after rerooting my phone on a freshly installed .340, it dropped me into a bootloader after the red motorola M came on.. Indicating that the gingerbread took but that there was an issue somewhere in the startup.. Eventually I gave up cause the phone wouldn't boot into recovery or anything and I ended up doing the same thing as you looking to SBF.. Well apparently somethings going down here thats causing a problem with RSD lite..
Heres what I did:
Turn the phone to a bootloader, check the status make sure everythings okay
Connect the phone to the computer make sure no drivers start installing
Put the SBF into your C:/ directory, only C:/ So its basically C:/SBFfile.sbf
Now go into your motorla C:/Program Files/Motorola/RSD Lite:
Find any logs, Flash error logs. Flex logs. Anything that is a log, and delete all of them.
Run RSD lite as an administrator.
Try the SBF process from the C:/ drive.
IF it doesnt work:
There is an error in the software that causes a problem with long name SBF files, some kind of simplistic programming error.
Remember everytime there is an issue to go back to your RSD lite directory and delete all logs.
If you didn't get it to work after the initial log deletes, and its still giving you problems
Rename the SBF to a1.sbf
Try the process again
Rinse and repeat [Clean the directory of logs, close software open it back up Try to SBF again]
IF this doesnt work a12.sbf
Rinse and repeat
so on and so forth, you will usually only make it to 8-10, that was what I had to do, eventually the SBF takes with one of the names, make sure you constantly delete the logs there is some kind of character rejection and you need to match the name with a certain length requirement specific to your computer.. i hope this helps.