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Checking, it seems the ST6000DM003 is a SMR drive. I don't recommend putting it in any pool where you want decent rewrite performance, but if you're only wanting good read performance it's fine.

... Would that 6TB USB HDD happen to be a Seagate Backup Plus Hub by any chance? Because I bought one a ways back (P/N 1XAAP2-501) and it behaves exactly the same way you've described. The drive inside might actually be okay, just with a lemon enclosure. Since yours is out of warranty and you're planning to ditch it, consider instead shucking it and using the HDD as an internal drive (run the Seagate long test on it again of course). BTW just FYI, Seagate has used SMR drives in its Backup Plus Hubs and I don't recommend using SMR drives in a pool, so also check the part number of the drive itself to see if it is one, with (if you don't want to open it up to check) a utility like Crystal Disk Info or similar.

I think a 2nd Pool for non-critical data using old hardware is an excellent idea.