Running a software-based RAID 5 with this thing is a very bad idea.
It also sounds like you have made a poor choice of drives.
Why? The failed one so far did not fail completely -- using smartctl I was able to cause it to remap the bad sectors and have not lost data. The cause of the failure is also likely, to have been heat, which is why I don't want to be running fanless.
Drive manufacturer's often don't send identical drives as replacements.
That's Ok, as long as the replacement is of same or larger size.
Running a RAID 5 with a missing/failed drive for any length of time is asking for trouble.
Unless there is a danger of a correlated
failure (disks dying for the same reason), the risk of another RAID5-member going bad within even a few months of the array going into "degraded mode" is lower then me being struck by lightning on the way to work.
In fact, with proper cooling, it is quite improbable, that even one drive will fail, before some new technology obsoletes the entire arrangement with the whole 2Tb, that my array will hold, fitting on some Purple Ray DVD...
Now, can we go back to the question of a fan-replacement?