So, I got TimeMachine working by formatting the USB drive FAT32 instead. That seems to run stable for the last two days. However, that doesn’t solve the erratic behavior of mounting HFS+ between the two USB ports.
I’ve formatted another drive (USB stick to be exact) HFS+, turned off journaling and couldn’t mount it rw on either port.
Apparently, others are seeing similar issues: Mount read-write HFS+ drive in OpenWRT