[applemacbook]

Can I replace the hard drive in the 21" iMac MC508LL/A with an after-market SSD?

Will I have to use a 3.5" mounting kit for the SSD?

[Sebastien]

You should be able to replace the original drive with a similar form-factor drive of your choice. I believe the mounting kit is for adding a second drive (notably in the 24" iMacs).

[shifuimam]

It's possible, but de-dusting the LCD to put the glass back on is nearly impossible.

I wouldn't really recommend trying it, to be honest. It's an inordinately tedious job.

[applemacbook]

Originally Posted by shifuimam 

It's possible, but de-dusting the LCD to put the glass back on is nearly impossible.

That does indeed sound very tedious

[shabbasuraj]

The HD temp sensor on the oem drive might play havoc with your fans.

[AKcrab]

You would have to short the sensor to make the fan stay off. Not exactly something I would recommend.