@reolhalcyon
On my Android tablet I’ve been having the exact opposite problem:
Obsidian
SD card: no problems
Local Drive: NO opening or creating of vaults with error messages
A long time ago - before Obsidian - I, due to lack of space, changed my default download location from LOCAL drive to SD card.
So just a moment ago I switched back and made my Local Drive the “default
download” location again.
After a reboot Obsidian worked just fine on my Local Drive as well as still on my SD card.
I’ve switched back to my SD card being the “default download location” and Obsidian is still fine with both locations 
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So if you are having a problem with Obsidian and the SD card on your Android phone, I would try the following:
Open Chrome browser | 3 dot Hamburger menu top right | Settings | below Advanced | Downloads | click on Download location | click on SD Card
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After a reboot hopefully Obsidian will now work on your SD Card & still work on your Local drive.
Then later you can revert your “Download location” to your Local drive & Obsidian still should work on your SD Card.
PS:
I only had Obsidian on the Local drive on my Android phone - so when I popped in a USB stick earlier, Obsidian immediately asked for access permission for the SD card on my USB stick and thus both locations worked fine.