Please combine your two threads in one, its more easy.
So where's your backup? Ehh, your RAID1 you say. But this isn't really a backup, its live synchronisation. It lacks history; virusses, filesystem corruption, accidental deletions and issues in your computer system may all cause data loss even on RAID1.
So a backup is worth more than a RAID1. Having a backup seperate from the RAID1 is probably the best. But you can also not use RAID1 but use one of the two disks as backup solution, either automated (it syncs your work every day or so) or manual.
OVerheating is not really the killer for HDDs. Drives can work reliably at medium-high temperature (40 degrees celcius - 100 F) for extended periods of time. But in home cases the real killer is temperature variation; the temperature rises then drops again when you turn off the machine. This causes metal contraction and expansion, which i believe is the main cause for HDD failures.
If you pick WD Green drives however, or other energy efficient drives, you do not need any cooling as they run very cool. This is recommended if you store large files / archive stuff. WD Green disks are slightly less suitable as system disks (running Windows) though.
I just read also that it's not recommended to have the OS mirrored. Might have a rethink then - move one of the HDDs to a hot-swappable disk caddy and backup via some strategy daily (can then remove this to safe place in case of fire etc).
Heat is the main factor in the lifespan of any electronics, not just hdd's.
There's absolutely no reason to monitor hdd temps. If you monitor your internal case temps and they are fine then your hdd's are fine.
I bought an Aerocooler Modern-V to provide additional temperature monitoring. One of the sensors is measuring ambient (about an inch in front of the first CPU fan). Still, I am not convinced that hot spots will not appear. If you are going to use an external sensor, Seagate recommends placing it near the end of the drive, opposite end to the connectors. I plan on placing a second probe there. I don't expect the HDD to be a problem; just want to make sure. I don't plan on putting a fan there but could if, against expectations, it becomes an issue.
I also plan on monitoring the internal sensors (CPU, quad heads, GPU, fan speeds) using the usual tools.