Sluggish 2013 21.5" El Capitan iMac

My second hand 2013 iMac has been running really slowly.

I have already done a complete wipe and reinstall, but that didn't help.

trying to get to the bottom of this, and suspecting imminent HDD failure, I booted from a USB El Capitan installer, then ran diskutil in Terminal, and discovered a whole bunch of additional disk images.

dev/disk0 is the internal physical disk, formatted as GUID journalled, with the EFI partition at 209.7MB, and the main Apple_HFS partition.

dev/disk1 is my USB Flash installer, also with EFI partition of 209.7MB and the main Apple_HFS partition.

dev/disk2 is a GUID disk image (Recovery Partition?), with a 209.7MB EFI partition and a 6.5GB Install ESD partition.

Here's where it gets weird: Disk3 is also a GUID disk image, with a 2 GB Apple_HFS OS X Base System. These four devices show up in the GUI Disk Utility.

Then I have disk4 through disk16, with no type and named "untitled", 7 of which are 524.3 KB (disk5 through disk9 & disk13), 2 are 6.3MB (disk10 & disk16), 1 is 5.2MB (disk4), then 2.1MB disk3), 2 are 1.0MB (disk12 & disk15).

If I do diskutil info /dev/disk'n', I get the following Mount Points:

 /dev/disk4 - Mount Point: /private/var/log
 /dev/disk6 - Mount Point: /private/var/tmp 
 /dev/disk7 - Mount Point: /private/var/run
 /dev/disk8 - Mount Point: /System/Installation
 /dev/disk9 - Mount Point: /private/var/db
 /dev/disk10 - Mount Point: /private/var/folders
 /dev/disk11 - Mount Point: /private/var/root/library
 /dev/disk13 - Mount Point: /Library/Preferences
 /dev/disk14 - Mount Point: /Library/Preferences/SystemConfiguration
 /dev/disk15 - Mount Point: /Library/Keychains
 /dev/disk16 - Mount Point: /private/var/tmp/RecoveryTemp

I tried running fdisk -i -a hfs /dev/disk'n' (where 'n' is the number of the disk I want to remove), but I get "Resource busy" for all of them except dev/disk12, where I got "could not open MBR file /usr/standalone/i386/boot0: No such file or directory"…

I then get: ————————————————————————————————————————————————-- ———- ATTENTION - UPDATING MASTER BOOT RECORD ————- —————————————————————————————————————————————————-

Do you wish to write new MBR and partition table? [n]

How do I get rid of these additional disks?

TIA!

I smell an impending death of a hard drive. For that case, I suggest an upgrade to an SSD will do. Not only you have a newer storage device but also you have a faster computer altogether.

On a side note, if an SSD isn't your fancy, you can always purchase a new HDD, but go for the better models since it will benefit you more in the long run.