I just browsed eBay a bit and saw that older, used SAS drives can be had pretty cheap - 30€ for 4TB, but of course rather old drives, sometimes 10 years old.
Now, I wouldn't expect ultra reliable, ultra fast, super cheap drives here. But this offer seems compelling, even buying a spare drive for higher redundancy would still be pretty cheap.
Question is: am I too optimistic here? Are these drives bound to fail within 3 months?
SMART (the internal drive self-check/monitoring system, exposes a number of statistics that can be read by software on the host machine) exposes a "power on hours" counter and a "power cycles" counter - a high count of either of those would indicate a drive that had been heavily used. Also worth looking at the "pending sector count" and "reallocated sector count", as increasing values of those is a pretty good early indication of failure
Excellent, thanks!