Same issue here. When I try to create a vault on external SD card/storage, I get “failed to create vault. Unable to create directory, unknown reason.” Similar message when trying to use an existing empty folder on external storage.
It does allow me to create my vault in the Documents folder on main (built-in) storage, though. (Which is the workaround I’m using for now, although not ideal.) But not in the root folder of main storage.
Using Android 8.0 on Samsung Galaxy S7, Obsidian Mobile v1.0.3. Screenshots attached showing Obsidian does have all storage permissions that I’m able to see to give it, and system info. Also reports that Obsidian has “access to 5 folders/files” next to the listing for “External Storage”, but no way that I know of to tell which folders those are.
Also of note, the file picker dialog shows “allow write permission” at the top of its popup left bar menu when picking a location to place the vault folder.
PS, very grateful to the developers for making this available on mobile now as well as desktop; it works amazingly well and better than expected, even on my older devices (except, notably, when using iCloud as sync provider on iOS, (on iPhone 6s/iOS 13), app becomes practically unusable due to slowness, lags, hanging. But I switched to using Obsidian Sync, and everything works quite smoothly. Somehow iCloud sync slows things way down. )
So far, so good on Android with Obsidian Sync on this somewhat older device.
(Sorry to digress off topic, but wanted to share that I’m case it’s useful to someone else while I’m here, in case I don’t get around to posting it separately.)