After a full drive backup, new larger HDD has unusable space.

A little while back I had my computer in for a service and I got the store to copy over the contents of my ailing 2TB storage drive to a brand new 3TB drive. When I picked the system up I was told that the backup had taken the file table from my smaller drive and that a third of the new drive was unusable because of it, the damn store clerk didn't even expect this would happen, and of course still charged me for this botched backup.

How on earth am I able to recover this unallocated space? I completely emptied the drive, expecting I could just fix it through disk management or something but my drive appears to be permanently set to 2TB. I deleted the volume, but now I just have two unallocated blocks and they will NOT merge. I can't even convert the extra space into another volume, it just sits there unable to be used.

Someone please help, I'm tearing my hair out trying to make sense of this.

Thanks! Just got it done now after finding software to convert it, ended up using EaseUS Partition Manager.