I have a MyBook Studio 1tb for Mac (Snow Leopard 10.6.6) and tried to update the firmware with WD Firmware Updater v22.214.171.124.app
I get the same error message as the OP. Then after running your program, the WD drive is no longer recognized and has to be rebooted and plugged in again before Mac will recognize the drive. Was WD Firmware Updater v126.96.36.199.app tested on Snow leopard?
p.s., the VCD hider software didnt work either… I have been suffering through your bloody WD Smartware ■■■■ ever since getting this drive as a replacement for a failed WD MyBook.