The writing with OS X issue is caused because OS X tries to check how much free space exists before writing a file. On the Almond+ it is sharing the directory /mnt which is part of the root filesystem and the root shows up as 100% full (due to the way the rom & overlay combining works.) To get around this, Samba needs to share the specific mount points for each USB device that is plugged in. Windows doesn't verify the amount of free space before writing a file, so it isn't impacted by this issue.
I modified a hotplug.d script to add/remove uci sambashare entries when a USB storage device is inserted/removed, and cause the smb.conf to be regenerated with a share specific to that mount. The (easier) alternative until Securifi releases their own implementation of this is to modify /tmp/smb.conf and add another entry similar to [share] but with a different name and the correct /mnt/usb/devicename path.