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?