Wouldn't taking the drive out of the My Book enclosure void the warranty on the drive?
Since transfer speeds would be capped to about 115MB/s over 1GbE and less over wifi, wouldn't it have made more sense to purchase a Seagate 8TB archive drive do do this? That way, you will still have the 3 year warranty on the drive. Newegg is currently selling the Seagate at $215.
The Seagate archive drive drops to 2-3MB/sec writes when it's nearly full - which is common with Time capsules(Shingled recording has to erase and re-record adjacent tracks that PMR doesn't have to do) and I'm not fully trusting in a SMR tech yet. Besides, it's really easy to open the case without destroying it with plastic picks and old credit cards. In other words, keep the enclosure for safe keeping and if the drive goes bad put it back in the case for warranty.
I picked up my first Seagate 8TB archive drive in March of last year. While they are certainly no speed demons, I never saw write speeds drop to 2-3MB/sec even when nearly 100% full. I currently have 8 of them in a RAID array and see about 300MB/s writes and 800MB/s reads.