Looking at the model number of your drive, this is Seagate, not HGST, which MAY BE why HGST diagnostic utility is not showing this drive I have a similar app that is called SeaTools (it's from Seagate). It displays even my new NVMe drive.
According to various sources and this Wikipedia article, pending sectors are these that weren't successfully read or written to. Outcome maybe one of these:
- the sectors can be subsequently read and return to normal pool
- the sectors can be reallocated and removed from the pool of available sectors.
Reallocated sectors (C4) value rising is a serious concern. I always write off a drive that displays either C4 or C5. Depends on the kind of data you have and how much it is valuable. I've seen a statistical research not so long ago, AFAIR from Google, that drives showing C5 are drastically more likely to completely fail in the next 6 months than normal drives.