... and it works. I put this code in /etc/fstab to automount at boot.
LABEL=32 /mnt/32 auto defaults,nofail 0 2
LABEL=16 /mnt/16 auto defaults,nofail 0 2
LABEL=8 /mnt/8 auto defaults,nofail 0 2
mhddfs#/mnt/32,/mnt/16,/mnt/8 /mnt/MPD/USB fuse defaults,allow_other 0 0
Some notes that may help others going this route:
In this test setup I set the label name of each USB/SD to its size and used the label for targetting, but fstab also supports other ways to target devices. The mounting only works if you first have created the mount point folders ( /mnt/32 and so on). I disabled usb automount in Rune UI to avoid duplicates in the Rune UI main library menu. If one of the usb devices is unplugged at boot mhddfs still works and simply unifies the space of the remaining plugged in devices in its list. One of the USB devices first had FAT32 format which caused problems in listing file/folders with non english characters. Reformatting to NTFS (and copying files over again) fixed that.
One cosmetic bug: the Rune UI source page reports incorrect size for the unified mount. It should in my test case have read 52GB but says 24GB IIRC. But Rune still adds all files from the unified mount to the library.
