I've tried flashing with a BIN file but MTKFlash immediately stops responding and I have to do CTRL + BREAK.
Here's what works for me:
DOS bootdisk: 6.21
MTKFLASH: Version 1.80
FW file: RPC1V050.BIN (from previously mentioned Dangerous Brothers download) - this is a region free mod of the BTC 50 level firmware
Command syntax: MTKFLASH 4 w /m /b RPC1V050.BIN
I have my BTC as Secondary Slave, hence the value of 4 for the first parameter.
You may want to jumper your BTC as the Secondary Master, temporarily remove any other drives on the Secondary IDE cable, configure in your motherboard BIOS as such, reset your system and boot up with the DOS bootdisk. This eliminates the possibility that some kind of quirk involving the presence of two drives on the IDE channel is causing your problem.
This might also be a good time to make sure your IDE cable to the drive is properly seated on both ends, and/or substitute a known good cable just to preclude that a bad cable is contributing to the problem.
If you do this, your command syntax would be:
MTKFLASH 3 w /m /b RPC1V050.BIN
where 3 indicates it is the Secondary Master drive