It can be done, but I’m not sure which SKUs it applies to, or that it’s particularly advisable.
Both the 2.9 GHz and the 3.1 GHz 21.5″ iMacs have space for a second drive bay, and there’s an iFixit tutorial explaining how to install a second drive. (iFixit usually tear down new hardware within a day or so of release, and have a lot of tutorials for things like hardware upgrades and repairs).
Currently their page for the 27″ model doesn’t have such a tutorial. However, there’s a page on their forums where people are discussing associated teardowns and the possibility of such an upgrade. The poster ifixit611 writes:
The 27″ iMac holds one 3.5″ drive (unlike the 21″, which uses a 2.5″ drive), and one “blade” SSD like the new MacBook Pros use. Unlike the new 21″ iMac, you can upgrade the RAM on the 27″ w/o cutting the machine open (there’s an access hatch on the back).
Replacing the disk requires cutting open the display and removing the left speaker. You’ll need a proper mounting bracket (e.g., an Icy Dock) to mount a 2.5″ SSD in place of the 3.5″ drive. Adding/replacing the blade SSD requires pulling out the main logic board.
They conclude that they won’t be performing the upgrade before iFixit or OWC provide a proper “kit”.
So it sounds like, in theory, you could perform this upgrade on a 27″ iMac, and the instructions from iFixit for upgrading the 21″ would probably be helpful in this regard. That said, hardware is not my area of expertise and I generally get twitchy about opening up iMacs. Do at your own risk, etc.
It is possible to create a Logical Volume Group spanning over more than two disks. I doubt that it will differentiate between fast and slow HDD. The SSD part should work though.
To accomplish this put all drives in your iMac and boot to a Mavericks or Yosemite bootable installer thumb drive.
Partition all drives as simple volumes with Disk Utility.
Then open Terminal and enter diskutil list to get an overview. The result is similar to this one:
/dev/disk0
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *256.0 GB disk0
1: EFI EFI 209.7 MB disk0s1
2: Apple_HFS SSD 255.6 GB disk0s2
3: Apple_Boot Boot OS X 134.2 MB disk0s3
/dev/disk1
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1000.0 GB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_HFS HDD1 999.6 GB disk1s2
3: Apple_Boot Boot OS X 134.2 MB disk1s3
/dev/disk2
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *1000.0 GB disk2
1: EFI EFI 209.7 MB disk2s1
2: Apple_HFS HDD2 999.6 GB disk2s2
3: Apple_Boot Boot OS X 134.2 MB disk2s3
/dev/disk3-disk15 with sizes between 1 GB and 512 KB are part of the boot disk as well as the thumb drive.
Now create the Logical Volume Group with:
diskutil cs create Fusion /dev/disk0s2 /dev/disk1s2 /dev/disk2s2
Choose the proper disk identifiers found in the diskutil list.
The output will be similar to this one:
Started CoreStorage operation
Unmounting disk0s2
Touching partition type on disk0s2
Adding disk0s2 to Logical Volume Group
Unmounting disk1s2
Touching partition type on disk1s2
Adding disk1s2 to Logical Volume Group
Unmounting disk2s2
Touching partition type on disk2s2
Adding disk2s2 to Logical Volume Group
Creating Core Storage Logical Volume Group
Switching disk0s2 to Core Storage
Switching disk1s2 to Core Storage
Switching disk2s2 to Core Storage
Waiting for Logical Volume Group to appear
Discovered new Logical Volume Group "0AD5A644-34A7-4B44-A9BC-CD37411B8910"
Core Storage LVG UUID: 0AD5A644-34A7-4B44-A9BC-CD37411B8910
Now create a new Logical Volume with:
diskutil cs createVolume lvgUUID jhfs+ Fusion 100%
with the example above this is
diskutil cs createVolume 0AD5A644-34A7-4B44-A9BC-CD37411B8910 jhfs+ Fusion 100%
This will yield the following:
The Core Storage Logical Volume Group UUID is 0AD5A644-34A7-4B44-A9BC-CD37411B8910
Started CoreStorage operation
Waiting for Logical Volume to appear
Formatting file system for Logical Volume
Initialized /dev/rdisk16 as a 2 TB case-insensitive HFS Plus volume with a 172032k journal
Mounting disk
Core Storage LV UUID: 34345C6A-421B-4F03-A6D1-4C5A144FA610
Core Storage disk: disk16
Finished CoreStorage operation
The 2 TB is erroneous: in fact a ~2,25 TB volume will be built:
...
/dev/disk16
#: TYPE NAME SIZE IDENTIFIER
0: Apple_HFS Fusion *2.3 TB disk16
Logical Volume on disk0s2, disk1s2, ...
34345C6A-421B-4F03-A6D1-4C5A144FA610
Unencrypted
...
Now install a new system and enjoy your DIY SuperFusion Drive.
One fact cannot be concealed: the SuperFusion Drive is even more vulnerable to drive failure than a normal Fusion drive. The failure probability is comparable to a RAID0.
Here is a comparison for 1, 2 or 3 disks:
Survival probability (for new disks and being optimistic):
Nr of disks 1st year 2nd year 3rd year 4th year 5th year
1 98% 90% 83% 78% 73%
2 97% 82% 69% 61% 54%
3 95% 74% 57% 47% 40%
Considering that at least one drive is already 5 years old the survival probability is even worse. So better get a reliable backup drive!
A deleted question invisible for some or most users asks for a speed test. A (Super)Fusion Drive is no RAID0/5/6, so r/w speed directly depends on the physical disk written to/read from. Usually at least 4 GB of the SSD are kept free. One can expect that the first ~4 GB written to the DYI SuperFusion Drive are transferred with SSD speed (150-550 MB/s) and the rest with HDD speed (20-120 MB/s). The read speed depends on the location of the files (i.e. SSD or HDD) and is similar to the write speed.
Best Answer
You cannot prevent your mechanical drive from powering up.
It’s a standard 3.5” SATA hard drive with a power connector directly connected to the power supply. There’s no switch or command you can issue that will cut off the power to the drive. Repartitioning it and wiping it clean won’t prevent it from spinning up and doing its periodic reads.
The only solution is to disconnect it.
However, since you have to go through the trouble of cutting through the adhesive holding the glass, you might as well upgrade the drive to an SSD. I’ve done this exact procedure - it’s not difficult, just time consuming.