I seem to have succeeded using another app called Apps2SD
It does it using Bindings rather than symlinks. Apparently, a symlink (which Apps2SD offers too) requires that both source and destination are formatted with a unix file system e.g. ext4. An my SD card is formatted with exFAT… I could change that but it doesn’t appear necessary. But this could be why I get that error message above.
The binding seems to work but produces weird results in that the files end up in both places but the free space report (from say Root Explorer) for the source path is wrong because the files aren’t physically there. I am just testing it properly now, with far more data than could fit into the source path.
I do have a related Q though: when I come to replace the 64GB SD card with a 128GB one, can all the files just be copied over, say under Windows? That’s what I always did in the past with SD cards in android phones.
Result has been confirmed. Apps2SD works!
It works with a destination SD card formatted with exFAT, 64GB, single partition.
It’s pretty weird to see the Sony app download 20GB of data from the camera, to internal storage with 7GB free space, and with the data appearing in both internal storage and the SD card, but with the free space in the former remaining at 7GB 
A possible caveat is that the tablet’s file system had previously been modified with Link2SD as described above.
I think Apps2SD does a similar job to Link2SD when it comes to moving apps to the SD card. I have not yet tried Apps2SD for that, in case the two interfere with each other.
Update: APPS2SD works but the internal file system still (using the folder mount method) needs to have enough space for each individual file.
This is because the file is initially written (by the stupidly written app) to the internal file system and when the app closes it, apps2sd picks it up, copies it to the external SD card, and deletes the original.
So if e.g. you have 5GB space in the internal and 100GB space in the external, and are transferring the usual 4GB chunks, that works. But a 6GB file will not work.
I don’t know if real symlinks solve this. They need a unix file system at both ends.
Peter wrote:
I don’t know if real symlinks solve this. They need a unix file system at both ends.
If I understand the situation correctly, they will. I expect, however, that a symlink needs a Unix filesystem only where the symlink resides, not the destination.
One could also try a bind mount. Something like:
mount —bind /storage/emulated/0/DCIM/From\ Action\ Cam /storage/extSdCard/ActionCam
(not the swapped arguments compared to “ln -s”).
this may need installation of busybox, and using the syntax
busybox mount —bind /storage/emulated/0/DCIM/From\ Action\ Cam /storage/extSdCard/ActionCam
This command will not survive a reboot; to make it permanent on a normal GNU/Linux system, you’d put the following line in /etc/fstab:
/storage/emulated/0/DCIM/From\040Action\040Cam /storage/extSdCard/ActionCam none bind
On Android, well… it seems different Android variants ("ROM"s) put that file in different places. https://android.stackexchange.com/questions/14530/whats-the-android-equivalent-of-etc-fstab
Thank you Lionel.
I solved this issue by always carrying a laptop whenever I am using that camera. The camera can generate up to 256GB of data which needs to be transferred from it before it can be used again (I can’t easily swap out the SD card because the camera is in a sealed housing, with silica gel pads inside). My current laptop is an XPS13 with a 512GB SSD and I have a USB 512GB SSD to which I can transfer the footage. I carry that laptop on all longer trips anyway. On my last trip I ended up with 500GB of video…
The original issue existed really because over wifi the only way to get data off the camera is with the Sony app, and that idiot app is hard coded to access only the device’s internal flash, which is practically never big enough. Windows has the same problem over wifi because Sony made the wifi protocol secret (it is actually very likely discoverable if you know how to hack wifi – same issue exists with various DSLR wifi interfaces, and some 3rd party apps exist now for those).
But windows can browse the camera conventionally (as a hard disk) via a USB cable, for which I have a sealed connector so the housing doesn’t need to be opened. I don’t think android can browse it; it doesn’t support that USB device class.
Then, having copied the files over, I make room in the camera by reformatting it over wifi using a phone or a tablet, using the Sony app. The logical drive is R/O so the files cannot be deleted over USB! Sony “marketing” decided that, since the previous version…
Whoever designed this stuff should be shot – or be given the job of an unpaid forum mod for life
