How to move sync Storage, to a new piece of hardware (raspberry pi problem)

[bugsymalone666]

So first time posting, because I have come across a problem that I have been unable to resolve by searching.

For maybe somewhere around 7 years or so I have had a setup with 2 raspberry Pi sync servers, one at my home and one at my parents, meaning I can work on my pc at home, then when I get to my parents and use my PC there, files are sync'd. The actual setup had a number of PCs also so it was a bit like:

Raspberry Pi MASTER -my house
Raspberry Pi master 2 - my parents house
PC Master 3 - my house

Then lots of PCs/laptops round the place, either mine or my wifes that would have there own folder/shared folders, but each of the devices above had all folders on them.

So roll on the system running fine for a good few years before a snag, Raspberry PI MASTER which was the centre of the universe, for some reason ended up with an error or fault in the OS, stopping it from booting, so the project kind of fell apart a bit as I wasnt about to join new PCs to the existing share of folders.

I'm struggling to remember how I setup RPI MASTER and master2, I feel like they were cloned with their own hard drives, their content was identical though. So I thought what makes the Pi know what it is, well apparently Sync puts an ID file somewhere. This is where my confusion starts and where my need for help begins:

RPI MASTER - this had a 1TB WD 'MyBook'  external powered drive attached, I didnt reformat it so its Fat32, and on the root of that drive is a ./sync folder, which has an ID, ignore list etc

RPI master 2 - this has a much newer self powered 1tb WD mybook equivalent, but despite the setup being practically identical, there is not a ./sync folder on the hard drive. I think this drive is setup as NTFS.

So as I misplaced RPI MASTER's sd card it seems after trying to investigate the problem a few years back, I figured I'd just clone RPI Master2's, plug it in and because the ./sync folder is already on the attached drive (which I had mounted in FSTAB at boot and just had to change that) it would just go about its business and be what it once was. Nope! sync will come up as a web page, but it shows no folders or anything.

So the question is, how do I tell Sync where its folder should be, so it stops looking at the old one? then it can resume its original identity