Cannot access hard drive even though it appears to be there

Everything in your description indicates that the hard drive in question is defective and no longer works.

/mnt/84edaaff2c-c293-4a73-9436-93172f5563fd is just the mount point in your filesystem where the drive would be mounted if it was still functional. If the drive is unmounted, or doesn't get mounted in the first place because it no longer works, the mount point remains there as an empty directory.

The fact that the disk didn't show up when you connected it to a different computer via a USB SATA adapter is a very strong indication that it is defective. In the best of worlds you would have a backup of any files you really care for, and now would be the time to restore them to a new disk from that backup. If you do not have a backup and those files are really precious to you then you might turn to a data recovery service to try and rescue some or all of the data from the defective disk. That option is not cheap, though.