Replace Disc ... SSHD

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steveTu wrote:

Thanks - I appreciate and take on board your advice - but for future reference, can I ask what makes a drive more suitable for a NAS?

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There are some drive firmware features (for instance TLER) that make a drive better suited for RAID.  TLER (Time Limited Error Recovery) is generally disabled on desktop class drives.  NAS purposed (and enterprise class) drives also are supposed to control vibration better, which is helpful in a larger NAS than the Duo.

Some larger drives (> 4 TB) use SMR technology, and doesn't work that well in a NAS.  Sustained write speeds with SMR are quite slow, which is a problem when the array is synced.  That's not an issue here though.

Generally Seagate and Western Digital don't recommend their desktop drives for RAID (other than RAID-1, which happens to be what your Duo is doing).

Note there are folks (for instance BackBlaze) who routinely use desktop-class drives in their RAID arrays anyway.  Though I think that is (at least in part) a risk/cost tradeoff that they are making (using a mix of cheaper drives, and risking replacing them more often).  

In this particular case, the Ironwolf is also a bit cheaper than the Barracuda.  Plus the Duo isn't really able to take advantage of the faster RPM, since it's performance is limited by it's CPU. You'll also be limited by the other old disk (like another 5900 rpm drive).  The large cache in the Barracuda (the SSHD) could be an advantage, though only if both drives in the array had it - and I don't think it would make much difference in practice, since the Duo is caching in it's RAM anyway.

FWIW, I use NAS-purposed drives in all my newer NAS (generally WD Reds, though I do have some IronWolf), and find they are cost-effective, quiet, reliable, power-efficient and give good performance.   My own Duo has an old Seagate Barracuda (ST32000542AS) and a newer WD Red (WD20EFRX) - no need to replace the Barracuda, as it is still working well.