access to USB Drive

I have. Mac mini running Volumio and I connected a external drive to it with the intention of moving the files to the 2TB Volumio internal drive. Volumio see the drive and will play music.

On my iMac I can map to all the drives and can transfer music from the iMac to volume but as I am trying tp move 15000 files its quicker to do it locally.

Problem is I am refused permissions to access the drive from the Mac once I am mapped to it. I can see internal but not the external USB.

Now I know nothing about unix and I don’t even know how to open the terminal window to type any commands.

Is this really that hard??