Editing fstab to mount /data from a different partition

As you already expected, the way your question is phrased it looks like an XY problem: There are different ways to achieve your goal (mounting /data, /cache etc. from a different partition), while your question focuses on "editing fstab".

For a working solution, see e.g. Mount a folder from external sd as /data: What's described there should work on all devices/ROMs – while "editing fstab" bears some problems:

• While that's of course the way it works on Linux, we're talking Android here – which uses a Linux kernel, but a very different setup (see: What's the Android equivalent of /etc/fstab?)


• An fstab file is not present on many/most Android installations (see How to list all major partitions with their labels? for details)


• If an fstab file is present, it's usually part of the boot image (see initrd for details). This means, your edits to the file would be in vain: on the next boot where you'd expect them to be processed, instead the file would be overwritten from the boot image and then processed – without your changes

Verdict: Even if your device has an fstab file, you cannot simply edit it and be done. You'd have to rebuild the boot image instead – a task that goes beyond the scope of this site (which is end-user orientated, and creating/rebuilding a boot image rather is in the domain of developers). It might not be straight "off-topic", but at least border-line/gray-zone.