So a hdd failed a couple of weeks ago and I got a replacement. I think I've successfully downloaded everything to the new one.

I guess the answer to this is fairly obvious, but the one that failed was the internal H: drive. It's replaced with an external drive, which also now has the letter H:, looking in Backblaze at this -

I presume I just untick the unplugged H: drive and tick the unticked new H: drive and the process is complete ?

(Hopefully I'll shuck the new drive so it becomes the new internal H: drive, but that might take a while...)