I’m loving my 8 external drives, in fact about to buy 2 more.
Yes I think that internal can be a little faster but speed isn’t as big of an issue as you may think.
The externals that take 8 days (my Seagate 8TBs took about that long) are going to be SMR technology drives. Very slow to write, but perfectly acceptable for read, over and over and over again. I looked to see if your drive is SMR and I could not tell.
The best bang for the buck was Seagate external 8TB from Amazon $150. That is what I have been buying and perfectly happy with them.
Where are you sourcing your 8TB WD EasyStores?