Is the Hard drive Upgradeable?
As it is known, Apple was making harder to replace internal hard disks. What is the situation for the new thin iMacs? Do they still require disks with Apple ROM? If they don't, is there enough space for the higher capacity 12.5 or 15 mm 2.5" disks in 21.5 models?
Yes, you will need a Apple spec'ed HD to replace your current HD if that is your Q.
If you are thinking of adding a 2nd drive, Apple has indeed gone with a up-coming PCIe connection for the SSD port over their propriety SSD connection (still not ratified so it could end up being unique to Apple). It's intended to support a SSD not a HD. Besides the 21.5" model just doesn't have the space inside to hold a second traditional 2.5" HD. Even a SATA SSD will need to be one of the barebones types. The good news is on the 21.5" systems Apple has put the SSD connector in (if you buy a HD only unit) so you can add it in later (the late '11 models don't have it).
I'm not sure even if the new 'Thin' iMac 27" has any space to spare for a 2.5" HD either you might be able to mount one of the super thin drives just coming out or maybe a smaller iPod type drive but these thin or ultra small drives are not very fast or large Vs a SSD. At least right now.
From today's iFixit email, "users can now put in a second hard drive via the Fusion Drive SSD port, even if they don't pick the iMac with the Fusion Drive right out of the factory. That port is now PCIe, which should help get drives/adapters onto the market that will enable a second hard drive installation."
I have successfully replaced the Apple supplied SATA HD with a Corsair m500 SATA SSD, using the parts described in the teardown for this model and one of the previous 21 inch guides. No special cable was needed. The original HD rubber cradle is glued to the sides of the drive. It can be removed using the spudger from this site and then sticked to the sides of the replacement sata drive without any new adhesive.
The system fan is also still running at normal speed as before with the original HD from Apple (System has been up for hours with no increase in fan speed).
I am a little bit confused. Where I can find a compatibility list for PCI-e for the 21.5 iMac late 2013 i7, 1GB 5400rpm?
Unfortunately i wasn't able to find such a list and things are a little bit vague. Apart from being M.2 connection is there any other factor that may be conflict with the iMac?
In case I find a compatible PCIe M.2, is there enough room to add as a second storage device (actually I am planning to host the operating system on the PCIe) and use as a second device the standard 1GB 5400 rpm drive.