The Slice look like it uses a GL830 USB > Sata interface
That makes sense, it certainly looks like a USB drive from the way Libreelec and Kodi present it.
Maybe i will try the RPi2 build
I dont think it will make any difference for you with this issue. it'll possibly be useful when LE10.2 is released.
Maybe it's just a power issue
that would be my guess too, my original drive is rated at 1amp. When you connected it via usb, did you use a seperate power supply? The plug is rated at 3amps so the slice is capable of drawing plenty of power, cant find any real power output details for that chip.