Hi there, you mentioned that the hard drive not showing up on your Mac computer as well as the alternative Mac device, I speculate that it's due to the connection going wrong or the hard drive being failing, and the latter one seems more likely. The following fixes help the hard drive show up on a Mac: re-connecting the hard drive with an available adapter, force mounting it on a Mac, and checking Finder settings.
If all these solutions don't fix the external hard drive not showing up on Mac issue, then I can tell that the hard drive has failed. One thing you can do is replace it with a new one for further file storage or backup purposes.
It's worth mentioning that you'd better back up the hard drive regularly since hard drive failure and data loss happen at any time, which may cause big trouble if there is no backup.
Best wishes~