Bro dvpro, you are probably right.....as the issue is accessing the drive and not data corruption.
So, I'm back at square one....
your problem let me remember something same,
I had bought a "AS IS" Nuvi 765 from Ebay US$45 , "AS IS" description almost similar like yours, Computer refuse to enter into Nuvi directory, but SD drive is OK. I have spent time to tried so many cure but unsuccefully, until one day when my friend give me a spoilt thumbdrive, computer can't be open the drive directory, I formating the thumbdrive, then, I think formating thumbdrive, can make it work again, what about my Nuvi 765, so that I tried the same method.
after connect my nuvi 765 with PC, I can see the Garmin drive and SD drive, but when double click the Garmin drive, it Error like something corrupted, already forgot the Whole message. Due to many cure already tried but failed, then I tried to format the garmin drive with move the mouse cursor to Garmin drive, Right Click and selecting "format...", I prefer Quick format because I knew that while garmin drive formating, if halfway halt (eg. caused by Power failure, accidently plug out the USB cable..etc), it will make your nuvi drive permanent dead, I got a experience on my nuvi 205w, and also got same information from web, other people also has the same issued with halfway stopped formating. So that, I always prefer quick formating, just few second done.
After formating my Nuvi 765 garmin drive, my computer able to open the garmin directory, then I use the webupdater get back the necessary voice, text file...etc. and work perfect without any issued.
So that, if you finally can't find any cure from Expert here, and if you totally no Idea at all, you may try my method.