It seems that with the JB update, UMS has been disabled when trying to attach the phone to an external device like my stereo, before the update all I had to do was plug the USB cable in and I could play music stored on my SD card. Now all I get is a message saying "USB Unabaliable". I can still see the SD card through a Windows 7 machine as long as I change the connection type to "Disk Drive", but that only works when attaching to the PC, (which has the latest HTC drivers installed".
So is there a way to re-enable UMS for portablility to connect to the car stereo? By default, HTC MTP is the only protocol available...
Unfortunately, you can't. All the phones have been switching over to the MTP protocol. HTC in fact had both implemented together but completly phased it out with the upgrade.
Only thing you can really do is complain to HTC, Samsung, LG, etc..
Or hope that Sprint, Verizon, T-Mobile will request it be readded to existing phones for the time being.