IMac – I’m taking the 2010 iMac in for SSD installation. What is best way to secure the iMac

For whatever reason applessdstatistics started appearing for many non-SSD users after the macOS Sierra 10.12.2 update. Apple is aware of the issue.

As you've discovered, if you do not have an SSD it is totally safe to disable applessdstatistics with:

sudo launchctl unload -w /System/Library/LaunchDaemons/com.apple.applessdstatistics.plist

And no, there is no need for you to re-enable it.

[EDIT]

After reading the comments I thought I should add some additional info about this issue.

Based on the advice provided by Apple on 8 February 2017, there are actually two issues that appeared around the same time concerning applessdstatistics. I describe these below:

1. The first issue is the one directly referred to in the OP's question. This is the issue of Macs with only hard drives suddenly having applessdstatistics appear and start reading data from the hard drive (typically in 4GB lots). applessdstatistics is only supposed to run on Macs with a SSD installed, not on Macs with traditional hard drives installed. This issue has affected a large number of desktop users with internal hard drives, primarily older iMacs (although this may only be because there are more iMacs in the field compared to Mac Pros and Mac Minis). I am unaware as to whether it is affecting MacBook models with only hard drives installed (as Little Eden implies in their comment below), but I am aware of the following issue (see 2 below) which could be what Little Eden is referring to.
2. The second issue, however, does affect various MacBooks (and other Macs) which do have a SSD installed. The issue is not the presence of applessdstatistics and the fact it is running, but the fact it results in high CPU usage. applessdstatistics has been running in the background of Macs with SSDs installed for a long time without issue until macOS Sierra 10.12.2 was released. However, after this release it has resulted in high CPU usage and, in some cases, a noticeable amount of noise coming from the machine.

Notes:

• The 2nd issue (re high CPU usage and noise levels) is also affecting some non-SSD Macs affected by the first issue.
• It is my understanding that Apple has an open ticket on this issue (i.e. I think it's the one ticket for both issues as they're related) and that a future update will resolve this.

A Fusion Drive is a composite of two physical drives: a fast but small SSD and a slower but larger HDD. Each of the physical drives contains three partitions: an EFI partition, a so-called physical volume and a special volume. The special volume on the HDD usually is the Recovery HD.

The two physical volumes are part of a Logical Volume Group. Creating a default Fusion Drive means building one Logical Volume in the LVG spanning both physical volumes. The Logical Volume is the one mounted to root (and if enabled - the volume visible on the desktop).

A file stored on the Logical Volume may reside on the SSD as well as on the HDD. System files may also reside on both drives. Usually an algorithm ensures that system files are stored on the faster SSD but there is no guarantee.

Removing one of the physical drives means removing one of the physical volumes and therefore tearing the LVG as well as the LV apart. After destroying the LVG/LV the file system as well as system files or other files are corrupted and the remnants won't be bootable.

To replace either of the drives, backup your exisiting volume(s) first with Time Machine. Then check whether you can boot to Internet Recovery Mode or create a bootable thumb drive and also try to boot to it.

Shutdown your Mac and replace one (or both) of the old drives. Then boot to Internet Recovery Mode or the bootable thumb drive.

• Open Disk Utility and completely erase both disks (each to one volume/GPT/Journaled HFS and assign names e.g. "fusion1" and "fusion2" but not "Macintosh HD"). Don't erase your thumb drive or a drive visible called Boot OS X
• Open Terminal in the menubar Utilities > Terminal and get the device identifiers of the two hard drives by entering diskutil list. Booted to Internet Recovery Mode you will get a list of 14-16 drives - only two of them are your hard drives. Simply check the sizes. The drives related to Internet Recovery Mode have a size of 1.2 GB or smaller!

• Create a new LVG and LV (here I assume the 120 GB SSD has the disk identifier disk0 and the 3 TB HDD disk1 - they might be different though!):

  diskutil cs create Fusion disk0 disk1
  diskutil cs createVolume lvgUUID jhfs+ "Macintosh HD" 100% #replace lvgUUID with the UUID shown in the output of the previous command
  

• verify the Fusion Drive with:

  diskutil verifyDisk disk0 # use one of the disk identifiers found previously 
  

• Enter exit to quit Terminal.app
• attach the Time Machine backup drive and in the Utilities windows choose Restore from Time Machine Backup > Select backup source > ...choose the drive and restore it to your Macintosh HD volume.
• After the restore task has finished successfully, boot your Mac normally. Check if a RecoveryHD is present by opening Terminal and entering diskutil list.
• When the RecoveryHD is missing, simply reinstall Sierra with the latest Sierra installer downloaded from the App Store - which shouldn't affect your data but creates an additional RecoveryHD - or use the Recovery Partition Creator 4.