Some investigation

Thanks for the blog post & videos @ptorrone! TIL about FAT12, and found @dhalbert's excellent info in https://github.com/adafruit/circuitpython/issues/111

I poked at this some more this evening, since I was wondering if I could verify whether this was a FAT12-specific issue, perhaps by mucking around with the block/cluster sizes to change the number of clusters on disk (I didn't get that far).

I see the same "erase/rename drive" behavior with PyPortal, with it's 8MB of storage. It shows up on my Mac (via `diskutil info`) as `File System Personality: FAT16` drive, vs `FAT12` for Circuit Playground Express. They both show up as `Partition Type: DOS_FAT_12` 🤷‍♂️. So I suspect it's still a FAT12-ish drive, but I'm not positive.

I also found the [release notes for fatfs R0.13a](http://elm-chan.org/fsw/ff/updates.txt), which include:

> Fixed f_fdisk() and f_mkfs() create the partition table with wrong CHS parameters. (appeared at R0.09)

It looks like CircuitPython uses a version that'd include this problem. I'm seeing [R0.12b](https://github.com/adafruit/circuitpython/blob/master/lib/oofatfs/ff.h).

I *think* this is the fix: https://github.com/abbrev/fatfs/commit/5c7915feab986c915f2142753f6df2e1497aad30 I tried applying it & seeing if it made a difference, but I got the same "erase/rename drive" & crash iPad behavior (assuming I did it correctly).

Finally, I pulled a panic log (`"incident_id":"E764F68C-1BFA-4244-98BD-5743A32672A7"` for any Apple lurkers) from my iPad, and it included:

> panic(cpu 1 caller 0xfffffff015b6e624): Unaligned kernel data abort

And unaligned accesses seem very plausibly related to 12 bit data.

Start by reproducing the erase/rename behavior on the Circuit Playground Express and PyPortal with iOS 13, then inspect lib/oofatfs/ff.h and the referenced FatFs R0.13a release notes. Compare the partition and filesystem details with the iPad panic log; done means identifying a reproducible FAT or iOS-specific cause and documenting the evidence.

Written by the indexing model from the issue text.