Locating hidden 471GB of files in Public share - Help!

I have a My Cloud 6TB (White, single bay, 2.xx firmware) and I have two shares set up to back up two external USB HDD. One has 3.59 TB of files and the other has 2.2 TB (5.72 total), so the 6TB My Cloud should in theory accommodate them both.

But when backing them up (via USB port and the Backup tab of the My Cloud console), the second one fails the backup because of lack of space. I checked the shares and other than the two backup shares, I only have the default Public share. Public share shows up as empty when browsing in Finder (on my Mac) but on the My Cloud console (Shares tab) it says it contains 471GB.

I’ve read people on here have had a problem with WDSync not deleting previously backed up files, so I have sneaking suspicion there are some legacy files from a previous USB HDD backup – I have recently transferred a number of large files to and from the USB HDDs involved.

So first, does my hunch sound likely?

And if so, how can I locate the stubborn file, then remove them to allow for a complete backup?

Possibly relevant:

I use the Synchronise back up (as opposed to Copy or Incremental which both accumulate older files), so I believe it should should overwrite any changes rather than filling up with files.

The Public share folder has three folders inside which are Shared Videos, Shared Pictures and Shared Music. In the finder these show up as empty, but in the My Cloud iOS app it shows each has a folder within called ‘mirrored’, which seems to has an alphabetically ordered collection of folders inside, probably made up from the various media files backed up to the two shares. I have the Twonky media server (and as many of the other options) turned off, but I have had Twonky running recently. I had a look through the various settings to see if this was something I could switch off but couldn’t find anything. Could these media files be the culprits? Or are they just aliases?

I have attempted to find the 471GB by switching on SSH access, then browsing using Transmit (FTP client etc) but I don’t seem to have access and I’m not sure which password, IP URL and folder path goes where. I’m not that familiar with this side of things.

Can anyone help me free up the space?

I know I could do a full restore but then the backups would be wiped and would have to start again (it takes days) and then the problem could well come back when backing up in the future.

Here’s the 471GB showing up in the My Cloud interface:

Here is the contents of the Public share in the Finder:

And here are some iOS screen grabs of browsing the Public share in the MyCloud app:
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