How to mount external SD card at a fixed path in CM13?
I've found a reasonably easy way to do something close enough: just set the volume serial number to something fixed. This is easy enough for FAT32-formatted SD cards: just use Sysinternals VolumeID:

D:\&gt;vol D:
 Volume in drive D has no label.
 Volume Serial Number is 07F5-432C

D:\&gt;volumeid D: ECAD-ECAD

VolumeID V2.01 - Set disk volume id
Copyright (C) 1997-1999 Mark Russinovich
Sysinternals - www.sysinternals.com

Volume ID for drive D: updated to ecad-ecad


("ecad" for "external card" but there are other possibilities, of course)

Now my external SD card is mounted at /storage/ECAD-ECAD, which is easy to remember for typing in those apps which don't let me pick a path, and more importantly, can stay the same when I upgrade to a faster card, for example (if I repeat this procedure).

The only complication is SD cards larger than 32 GB, which Windows refuses to format into FAT32. The best tool to work around this is Rufus, an open-source tool with a focus on bootable drives, but it can also just format a large FAT32 drive.