Partition on External HDD won't mount

That’s a good point. If you have a partition that’s failed you might want to consider it a sign to migrate to a new drive. If I had a failure like this one I’d probably replace it, under warranty or not.

Modern drives with high data density and spin rates have definite lifespans. A while back Backblaze looked at the drives it was using and found a jump in failure rates of between 9% and 17% between years three and four of their drives running 24/7, with an average annual failure rate of almost 12% by Year 4.

Because drives inevitably fail I replace working drives every five years.