Mounting sd card fails (BTRFS extref option of recent kernels)
Mounting a microSDXC card formatted as a raw BTRFS block device fails:
[ 9706.256553] mmcblk1: unknown partition table
[ 9706.524003] device label Jolla devid 1 transid 4 /dev/mmcblk1
[ 9706.526780] btrfs: disk space caching is enabled
[ 9706.526811] BTRFS: couldn't mount because of unsupported optional features (40).
[ 9706.572652] btrfs: open_ctree failed
Recent Linux kernels enable a BTRFS feature called "extref". I think the current Sailfish kernel doesn't understand this option yet; it might be a good idea to take this into consideration in future updates.
The card is a Samsung 64GB Pro microSDXC that came initially formatted as ExtFS. I reformatted it as BTRFS without partitions with a recent kernel on a Debian system. BTRFS apparently enables the "extref" option by default these days.
I hope this helps others who have the same problem.