Unable to find what's taking up so much space

You said it yourself: "sdb1 was unmounted due to a power outage". This probably means that in this time, some data was written to /mnt/hdd2 while sdb1 wasn't mounted.

Unmount these devices:

/dev/sdb1       1.8T  1.6T  108G  94% /mnt/hdd2
/dev/sda1       7.3T  6.8T  504G  94% /mnt/hdd

And then check the directories for data (starting with /mnt/hdd2) - I believe there will be around ~750 GB of data here.

My guess is that data was written to one of these directories, where the disk wasn't mounted. This data now "disappears" when the drives are remounted.

This is only a qualified guess, but 99% of the time when data mysteriously disappears, this seems to be the issue.