I have fans with 600 rpm - 2000 rpm, they start at around 500 rpm.
I think it's better to choose PWM fans that starts above 600 rpm.
Like Doyll said avoid fans with closed frame holes like the Cougar Vortex HDB and CM Excalibur.
I'm just curious, is the stock fan spinning at least around 900 rpm (+/- 10%)?
I have an Asus board but mine will show an error only if the CPU Fan is running below 200 rpm.
I can also change this value or just disable the speed alert in the bios.
The fan rpm is stated as 900 ~ 2,000 ± 10%.
The bios option CPU Fan Min Duty Cycle is by default 20%.
Try to set it with a higher value like 50% or 60%.
The fan header will supply a higher fan power (pwm signal) then the fan should start at a higher rpm.
One way to check the max rpm is disabling the bios option CPU Q-Fan Control.
The fan will spin at full blast.
Motherboard fan headers are tipically rated 1A / 12W.It would be fine to hook fans in the same header since the combined amperage / wattage doesn't exceed that rating.There's 2-way splitters like this and this.Some splitters like this or this draw power from the PSU then there's no risks to exceed the fan header power rating.For splitters powered by the PSU the speed control will work only by PWM.Not all 4-pin headers are PWM.For most boards only the CPU Fan header offers...
The 212 stock fan has 4-pin then it can be speed controled by PWM when plugged in the CPU Fan header.The AF12025's exist in 2 versions with 3-pin and 4-pin PWM.The AF12025 in the link is a 3-pin fan then it can be only speed regulated by DC.If the CPU Fan header doesn't offer speed control by DC then 3-pin fans will spin at max speed.
That doesn't sound working as the board manual described.
At least the CPU Fan should control 3-pin and 4-pin fans by DC.
Check in the bios the CPU Smart FAN Mode options: Auto, Voltage and PWM.
Set it as Auto.
Check also the CPU Smart FAN Control options: Normal, Silent, Manual, Disabled.
I would let it as default Normal.
After the required restart enter in the bios again.
Check the fans speeds.
Hope this settings will make the 4-pin headers working with 3-pin and...
The Cpu/Opt Fan can control only 4-pin fans.
3-pin fans will run at full speed.
The Cha fan 4-pin headers control both 3-pin or 4-pin fans by DC.
The CPU Fan is designed to control the heatsink fan.
The Opt fan is an extra header for the heatsink fan or case fan.
The Asus Sensor shows the CPU Temp, board temp and the fan speeds.
I would recommend to ignore the Asus Cpu Temp.
It's not the core temp and under heavy load the sensor could still show lower temps.
I personally like both brands and have used their hard disks.
I'm a Seagate 7200 RPM SATA III owner and my vote this time goes to Seagate just because I'm not from US and Seagate offers local rma with faster replacement in my country.
His products have also better avaiability and prices in my county over WD's.
I've never used the WD blue and black but I've used the WD green for years that cames with in the HP's desktops.
Both Seagate and WD Green run quietly and cool.