good that it is working now. If not realy needed to connect the HDD to another Windows based computer, I would recomment to use ext4 as file system format.
If you really like you can follow this linke to share a coffee 
https://dietpi.com/contribute.html
Great that it works now.
It’s either the Raspberry Pi kernel or the Debian dosfsutils and/or mount packages, it does not play well with. It’s obviously not auto-mounting (you mean systemd automount feature), but mounting such drive in general 
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Another question, did you format the drive on Windows? Probably formatting it on Linux/DietPi would have worked. I’ll keep the case in mind if something similar is reported again 
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Well, after configuring other applications through DietPi, and subsequently having to reboot both Pi’s a few times, the issue is back. At this point, I don’t even care - I am leaving it as is and will deal with the issue if the Pi’s are ever rebooted.
To answer your question, I did format the both SSD’s using the DietPi-Device Manager tool since I figured this would net me with the best format configuration, but alas, it did not fix the problem. Either way, I am leaving them both formatted as NTFS and will handle issues as they arise.
Also, I contributed via the link that you provided - hopefully you received it!
Thanks again to both of you - you truly went above and beyond in providing me assistance through this issue, regardless of the outcome. I learned more than I expected. Cheers!