I have a LaCie Rugged external HD that has had issues with the password for the disk-level encryption resulting from being formatted in macOS Recovery for use with Time Machine. On a number of occasions, when I needed to enter the password for various housekeeping purposes, on a given day the password would suddenly fail to be accepted, and I'd sometimes be able to unlock the disk normally if I went into the startup recovery mode -> Disk Utility to unlock and change the password, but on a few occasions I had to give up and reformat the disk to get it to work normally.
I have no idea why I persisted in using this disk, but nonetheless...
This has happened again, and since a few months went by between the last time I hooked up this drive and my most recent attempt, I have been second-guessing myself and wondering if I'd changed the password after taking sleeping medication/etc. some evening and forgotten it, but that would be extremely unlike me and in light of this drive's history, the former explanation seems to fit better.
My current situation:
I have an entire Time Machine section on this drive as well as a user-managed folder for storing large or unused files/docs, and have been confronted by the shuddering password query box each time I try to access it whether during a normal desktop session or in recovery mode.
But, when I tried booting my mac to an alternate startup drive (holding Option key at power-on), then selected this drive, it accepted my known password, ran through a boot cycle, then ended up in recovery mode. If I had indeed potentially forgotten the password for an encrypted Time machine backup on an encrypted HD (and for which account FileVault had been enabled), then I would truly be out of options and have to give up or try to brute-force the password through a list of potential password strings, BUT can this be reason to think that there is instead some kind of problem with the disk or its firmware, and that my password is still valid?
Given this, how can I safely access the files on the LaCie? The successful startup boot to recovery mode was unorthodox and didn't seem to work completely, since I still went to try to Restore From Time Machine, and it let me view the lists of backups by date, but when I selected one, it showed there was nothing visible in any of the folders... Of course, if the system was using that very HD as the startup disk, maybe it makes sense that it didn't work.
I need recommendations and some differential diagnostics from those with a higher level of expertise - I am already beyond my competency and am just a capable dilletante attempting viable solutions as I learn of them. Please forgive my rambling and self-indulgent prose record of the circumstances, but I wanted to be thorough.
Thank you very much for taking the time/energy to review, rebuke (I'm sure), and remand.