OK, thanks, and understood about the slowness.
It does sound like the drive may be failing, which could definitely cause that behavior.
If you are simply going to trash the old one, you could disassemble it and remove/destroy the drive. If you are going to give it away, sell it, or repair it, then I don't suggest taking it apart, because the internals are easily damaged.
iFixit - Mac - Repair Guides can be helpful in removing the drive, once you know the model. But again, it's not recommended if it will be reused.
C