Update:

After doing some testing, I've been able to unplug the drive as soon as the router self reboots. This has preserved the file db file and let me copy it off the drive.

There doesn't seem to be a size limit for the files db file, or at least if there is it's not consitantly crashing the router on reaching a particular file size. Largest files db file so far is 15,252KB, however I've had the router crash with it at 14,528 or lower.

I've tried deleting some files, deleting the last files it crashed on (seen in file db) in case they're corrupt, etc. and so far the best I can come up with is that there may be a maximum indexable file and / or folder limit.

It doesn't matter if it's movie, image or music format files, eventually and at about the same scan duration, the router reboots, the media_server folder is emptied and the scan process starts again.

Is there any validity to that?