Confusing problem with Drivepool

Hi :).... I think I've done something a bit stupid and don't know how to fix.

I have a external hard drive on my Drivepool and it had lots of spare space on. I detached it and connected it to my downloading pc and transferred data to it, with the idea of Just adding it to the Drivepool virtual drive once back connected.

When I connected the external drive with a good 300GB of data back to the pc the files on the drive are not there!.... It's showing as unallocated data, I think it's called!, being part of the Drivepool drive the files have somehow disappeared

Can you recommend a fix to either delete them or view them?

Thanks 🙏