2013 21.5" El Capitan iMac has loads of extra disks

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 inteal physical disk, formatted as GUID joualled, 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, the Recovery partition, with the 209.7MB EFI petition 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).

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!

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