Pulled the drive and ran diskcheck on a windows machine. Windows showed errors as soon as I plugged it in. Let the checker run and it seemed to clear it up. Drive is back in the server and we just booted successfully. So hopefully that's it! But I will be using the My Servers auto backup now in case this drive does give out.
So it appears that formatting to remove type 1 protection wasn't enough -- but also changing to 4K sectors appears to have made it work. Not sure if they are both necessary in any particular order but it wasn't until the 4K change that I was able to see the full size of the drive. Now it is assigned in Unraid and rebuilding parity.
The thread linked in the previous post does have a lot more detail about this whole process, but in summary it appears you can prep these specific Oracle/Sun HGST drives for use with Unraid using the sg_format commands together:
sg_format --format --fmtpinfo=0 --size=4096 /dev/sXX
--format will erase the entire disk
--fmtpinfo=0 will remove Type 1 Protection
--size=4096 will set sector size to 4K (instead of 512 or 520) [NOTE: your disk needs to physically support this change; an older drive might not]
/dev/sXX should be changed to your drive's device name
