I'm trying to accomplish something fairly odd, so bear with me.
I've added an external SD card to my old Droid Mini (basically just for an experiment to see whether or not I could expand the storage capability). It's currently running Lineage OS 13.
The issue is that the external SD adapter is very power hungry, and the kernel on this version (3.4.42) doesn't properly support autosuspend.
I can force the device into low power mode by unbinding the SCSI device from the USB device, but obviously it disappears if I do that.
I read that bind mounts can preserve the directory structure of the source folder even if it is removed, but I can't seem to get that to work (see here - "It is possible to unmount /some/where, in which case /else/where remains mounted.")
The idea is to wait until the USB device goes idle, then manually suspend it by unbinding it until it is needed again. However, if I unbind the device, the "stat" file I'm using to monitor idleness ALSO disappears, do I can't use that to check if activity is requested.
So my (maybe silly) idea was to make bind mount of the external card to a "virtual" USB device which doesn't disappear when the source is removed, and re-attaches it when it's needed.
Baby steps though, any ideas how I might go about preserving a "fake" drive for the system to operate through? Bind-mounts don't seem to do quite the right thing here.