[One (M8) Android Development] dragontology:
Not asking an ETA. However, I am running another Sense ROM, that has been updated to Marshmallow. Because I like to try the new shiny. ;) Anyway, there be dragons. Marshmallow has some serious issues with the SD card. And the developer of that ROM takes the position, "not my circus, not my monkeys, HTC made it like that so there you go." And honestly I'm not sure a ROM developer can fix it. It might just be How Marshmallow Is... or worse, How Android Is from here on out. And this ROM was ported from the unlocked version... it's the same thing y'all are gonna get outside the US. It's been ported to Verizon and Sprint but I doubt the carriers will fix it.

The first issue is permissions. Apps will be required to request permissions from the user, but they'll have to be designed to. If they aren't, they don't get them, except Internet which Android guarantees to all apps as of MM, so no blocking ads in games by denying Internet permission. You can't. So some apps won't ask permission, and won't get it, and will act badly.

One of the permissions is writing to the SD card, and reading it. Without that permission, no SD card access, it's like you don't have one for that app. The current FX File Explorer for example can't see SD cards. (They have a beta that can, should be going stable soon.)

You can't restore Titanium backups in Marshmallow yet. You can restore the data, but not the app. So what you do is, hop on Wi-Fi, download all the apps from the Play Store, then restore data where applicable. You can restore system data where you could before, like text messages. Titanium can't even see the SD card without some hackery. To get that done, you go to Menu → Preferences → Backup folder location, and then above the folder path, you tap Storage provider. Then you change it to DocumentProvider storage. That opens Android's recent files app. Close the left menu, tap three dots, click "Show SD card." Select it, then the Select/Okay button in the lower right. Then you can browse to the folder. But again, it can't restore the apps, only the data. And of course that's only if you're using a microSD card. It will see the folder just fine if it's in internal storage. Still can't restore from it, though.

SD card is mounted to a completely different place. Remember 8-digit serials for drives from MS-DOS (ABCD-1234)? They're back. For example, /storage/7DA8-6410. That's the mount point for my SD card. No more /storage/sdcard2 or whatever it used to be.

Format as Internal Storage — it's a trap. New Marshmallow feature, otherwise known as Adoptable Storage. Hides your internal storage, uses your memory card as internal storage. They don't add up. So your 80MB/s EMMC5, you don't get to use. 32GB of storage, you can move apps to it, but you can't use it for photo/media storage, downloads, etc. So say you have a 64GB card. You won't have 96GB internal, you'll have 64GB, and it's slower. microSD cards may advertise 90MB/s (like mine does) but the SDXC controller on the Snapdragon 810 (we only have the 801) throttles it to 10MB/s. 801 is probably less. So you get a little more storage (not a lot more) and it's slower. I would think even with a 128GB card, the tradeoff would not be worth it.

On top of that, the only microUSB flash drives Marshmallow can read are ones it's formatted. It can't read flash drives formatted by Windows or older versions of Android. So you want to put some files on your phone via flash drive, it wants to format the drive first. Which erases the files. So you do so and put the flash back in the PC. Windows can probably read it, write to it, and then you're good, but if you want to put it in another Marshmallow device, maybe it's going to want to format it (I'm not sure).

I'm trying his new version, but it mainly addresses connectivity issues. And I'll happily try ViperOneM8 5.0.0 when it comes out. But I'm not asking for an ETA. I don't even want it to come sooner than it's ready. If it takes a whole other month to get right, hey, awesome, that's great. Marshmallow is not unusable with a microSD card, but gone are the days of clean flashing a ROM, restoring from Titanium, and being up and at 'em in half an hour flat. More like half a day now. Though YMMV based on the apps you use and whatnot.

But, when ViperOne does support Marshmallow, I hope you guys nail it. If not, no worries. I'll go back to 4.6.1 and be perfectly happy there.