Unfortunately, LrM currently only handle the whole file that is imported into the App.
It cannot link to another file system. Here is a copy of what I wrote on another
This may be of interest:
A RAW workflow on the iPad - my initial impressions
The trouble currently is, to get to Lr Desktop, they have go via the Cloud so you have to regularly be near a good internet connection or have an iPad with loads of storage space (the photos can sync at a later date so it's best to leave them on SD also).
There is a request in for LAN syncing that you can add tour vote to:
Other users do use the iPad but development wise it's in its infancy and also down to choice if want to deal with all the negative aspects.
If you want your iPad it's best to use the Lr Mobile App, but unfortunately they have to go to the Camera Roll first.
Hope this helps in some way.
Thanks selondon for your quick reply. Bad news ;-(, Ive voted for this feature under your link! Hope Adobe is working on this and will release such a feature in the near future...then I need to find another solution, probably I have to move away from LrM, and thats not just ****...
I assume there have been no advancements on this? I bought an iPad Pro with Lightroom mobile as an alternative to a laptop for disconnected work with a wireless external hard drive, but so far this workflow is painful. The iPad Pro does not have enough storage (I have the 512) so I need to work off an external storage device, but I can’t seem to edit directly on that hard drive or even manage multiple files having to load single files one at a time, then copy them back.
Nope, no advancements that I know of.
As you may/may not know, Photos have to be imported into Lr Mobile (via the camera roll) and this takes up space within the App on the device itself.
(It doesn’t work from a HD like Lr Classic).
A real problem with a disconnected/travel workflow is the internet connection. As soon as the Photos are synced to the Cloud you can delete them from the Camera Roll (you could probably do this as soon as they import into LrM, but it’s best to be safe).
Lr Mobile self manages its space over time (reducing the local originals) but you can try doing this quicker by Clearing Cache within the app, once uploads to the cloud are complete.
Right now it seems like the biggest thing we’re waiting on is apple for updates like alowing apps to access the files on the SD card reader and allowing files from an SD or other storage (with the lightning to USB) to import into the files app on iOS 11.