[Transformer TF300T General] ozkrtech:
My Last Resort but worked 100% with me.

Heres the link ---> http://xdaforums.com/showthread.php?p=45120193#post45120193

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I have the same on my TF300t and A700 - but it seems i just managed to get my TF300t back

i tried all fastboot (reflashing recovery, boot, system etc) no dice - especially since writing system was done after 3 secs all the time.

Here how i restored my TF300T JB

download your appropriate lates firmware from Asus (WW, US etc.) and extract the zip twice so you have your blob file

install your device drivers if needed (i used Google SDK ones) (and as a Tip Win8 sucks for Fastboot stuff so try to get Win7 Vista XP for this)

Reset your device with a paperclip/needle (about 2 cm down of your sdcard slot) and hold vol-down to enter fastboot

(the folowing i did at least 10 times already but didn't erase misc)

fastboot erase system

fastboot erase recovery

fastboot erase userdata

fastboot erase boot

fastboot erase misc

fastboot erase cache

fastboot -i 0x0B05 flash system c:\adb\TF300t\blob (this is my path - your's may vary depending where you stored your blob) ((also this was the first time i saw the loading bar while flashing)*

fastboot -i 0x0B05 reboot

* it also took a lot longer as it should:

C:\>fastboot -i 0x0B05 flash system c:\adb\TF300t\blob

erasing 'system'...

OKAY [ 1.695s]

sending 'system' (800935 KB)...

OKAY [133.121s]

writing 'system'...

OKAY [153.973s]

finished. total time: 288.789s

im now setting my tab up to get back to CM (which initially caused this sh1theap due to autoupdate error, same as my A700 got bricked )

hope i could help you out!

cheers

-Buster <----(credits to him)

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This part was me now -ozkrtech <--- (credits to me)

In addition to that, if you simply can't see your device using the command "adb devices" go check on device manager if you see a device called just "Transformer" with a yellow icon indicating it needs a driver, and manually update driver, select "browse my computer for drivers" and then select let me pick from a list of device drivers on my computer" there you will see a list of like 3 different drivers, one of them says "bootloader" and one of the others says "adb" (i think they self explain) but just in case try with each one of them you can keep on switching drivers until you get your device recognized by the command "adb devices".

You should be asking yourself... whats the right moment to do an "adb devices" command? well my tablet same as -Buster's tablet had the wrong recovery menu flashed (wrong bootloader version) and it just kept booting into the corrupted recovery (in my case TWRP) with the "can't mount anything" and "what's the password" issue. When you get to password screen just hit cancel and try the "adb devices" commando and you'll see it gets recognized with your device serial number.

Once your device gets recognized by "adb devices" command you will proceed to use "adb reboot-bootloader" command and the tablet will reboot into the menu that had 4 options but now have 3 options and its missing the fastboot option (don't panic) in the last bootloader it is auto-fastboot mode when you reach that screen with just 3 options in it now (yeah is the screen with the RCK option) at this point try "fastboot devices" command to see if your device gets recognized same as "adb devices" procedure, it should get recognized with a different serial number (don't know why, maybe that number isn't a serial number), at this time you are SO LUCKY! now you will follow what I posted at the beginning from that other dude ---> (-Buster) or follow his link --- > http://xdaforums.com/showpost.php?p=44244313&postcount=12

And you'll have an unbricked tf300t in no time =) (I personally used the stock but newest Asus ROM) 4.2.1 (10.6.1.27.1) and it worked flawlessly.

**NOTE** don't turn off nor reset your device once you have formatted the partitions, no need for that (don't know what happends but it might get really hard bricked) just proceed to flash the stock rom after erasing the partitions.

I'm 100% positive this will work if your device is in same state as ours. (had TWRP for JB) then updated bootloader to version 4.2 and forgot to update twrp to version 4.2 or installed twrp version JB by mistake and lost fastboot and TWRP is asking password.

Hope it helps. Let me know your status and if it worked for you.