When your internet connection drops out, this doesn't hurt Bitcoin Core and it's data files. It's bad when your PC suddenly goes off. If you can detect a power dropout and your UPS is about to run low on energy storage, your PC should close Bitcoin Core gracefully and shutdown OS gracefully. That will keep Bitcoin Core's data files healthy.
Why did you start re-indexing? This shouldn't be necessary when Bitcoin Core shuts down normally.
With a 4TB HDD you have enough space to store an unpruned blockchain on your HDD (current sizes are ~446 GiB for blocks, ~5 GiB for chainstate and ~37 GiB for indexes/txindex (if you enable creation of transaction index)).
Pruning is only to reduce the size of the stored blockchain but has its own disadvantages as a rescan or reindex would likely always redownload the whole blockchain to read through it. As I assume you have enough space on that HDD, in that case pruning makes no sense.
If your current download is with pruning enabled, you would need to stop it, disable pruning and restart the sync of the full blockchain.
You can speed up the sync process of the blockchain if you set at least dbcache=2048 in bitcoin.conf file or to some even higher value depending on how much RAM your PC has. Don't overdo it with too high dbcache value (it's in MiB) compared to total RAM size and what else is running on your PC. With 4 GiB RAM, I'd try with dbcache=1024, with 8 GiB RAM dbcache=2048 (or 3072, maybe 4096 could work, too). With 16 GiB RAM you might be able to use dbcache=5120 which should likely be enough to hold the UTXO set completely in RAM during sync of blockchain.
After the initial sync you can reduce the dbcache value or comment it out by placing a hash # in front of it.