That depends on the controller firmware.

But if you want to be sure, just do a read-only badblocks test (if you use Linux). That’ll force the controller to read all blocks and (hopefully) rewrite those it finds to be weak.

Well, it should have given a warning that it couldn’t save due to lack of space.

I use cheap USB sticks if I want to hand someone some data.

Here, have what you want on a stick. No, I don’t need it back, you can keep it.

You need 2 drives, one as backup. HDs, no matter how good, can fail at any time without warning.