[Verizon Samsung Galaxy S III] HMkX2:
Thanks Captain_Throwback, your intel is EXACTLY what I need. Like I said, THANK YOU I can now provide explanations for all of your issues.

I think I have an idea what the "Fat32-partition-is-invisible" issue. Are you using Linux to partition and format the card? If so, you are not using a "regular" FAT32 format utility.

I am attaching a screenshot showing why my script wasn't recognizing your FAT32 partition. It seems the Linux "mkdosfs" command you were using leaves a nonstandard header signature. On the left is my FAT32 created using Windows (partition 3/made visible using BOOTICE), the one on the right is yours. It's completely different! (I have temporarily added support for mkdosfs in script 5.3 using a derivative of "blkid" identification I use for ext4)

I also know why BOOTICE "make visible" isn't working for you. If you are using something like GPARTED, it is creating LOGICAL partitions!! (Partition 1/FAT32 = the 1 "Primary", the rest are created under a "Logical"). The trick to make Windows see what partition you want relies on using PRIMARY partitions! (Primary = limit 4 max, Logical = raises cap to 16..... which really is not needed for a SD card. BOOTICE handles proper partitioning to allow its own tricks.)

You might have luck partitioning the card with BOOTICE, then "formatting" ext4 in Linux (or using MiniTool in Windows to do it all). BOOTICE really simplifies making and formatting more than 1 partition in Windows, and will probably solve your issue. (To format ext4, you can always stick it in the phone and use "make_ext4fs /dev/block/mmcblk1pX" command if you don't want to install MiniTool on Windows or plug into a Linux computer.)

UMS? Not MTP? That might be the problem right there. I never coded this script to intentionally support UMS. And I have no way to test UMS, since it is broken on the CM10.1 / Galaxy S3 I use. It sounds bad, but the fix is to use MTP in the meanwhile.

UMS treats your phone like an overglorified card reader -- and "unplugs" the card from the phone internally. Your exfat sd-card works WHILE it is initially connected to Windows, correct? When you click "safely remove hardware" in windows,

my script is not called to re-mount the card, so cannot apply the exfat fix!!!

. The phone acts if you pulled the card out and reinserted it cold -- and Vold does its shenanigans and messes it up.

You need to re-run my script ANY TIME the card is "ejected" from the phone and "reinserted", even if it is Windows faking doing that to get into UMS mode. If I knew how to code a receiver for AM broadcasts, and made this a GUI, I could make it automatic. But for now, you have to manually re-run the script every time you unplug/replug the SD card.

Finally, I know why you get the NOGO error. Run the command "mountpoint -n /system". If you don't see the words "mmcblk_____", but instead "by_name_blahblah", it means the algorithm I use to watch out for the system partition is not portable across phones. On mine, it says "mmcblk0p5". I trim that down to "mmcblk0" using REGEX, then say AVOID IT. I do not know why your HTC's "mountpoint" command works differently than mine -- it should be universal.

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I will post a temporary fix soon to all these problems.