Can't partition SDD with odd error

Question

Whenever I try to resize my OSX partition on my SDD from 251GB to 210GB, I get this error:

Couldn’t modify partition map because file system verification failed.

I've tried

• Booting into recovery mode and trying from disk utility there (repairing and partitioning)
• Booting into single-user mode and running fsck -fy like seemingly every website suggests

but whenever I try to do this in any situation, I still get this error. This is frustrating because AFAIK only OS X knows how to resize an OS X partition.

How can I get around this?

EDIT:

• Apple Diagnostics gave no errors.
• I first had this error when I had filevault (full disk encryption) turned on. I have since turned off FileVault and repaired any errors I caused by trying to partition the encrypted disk with Disk Utility in recovery mode. I have checked again that FileVault is off, and it is.

EDIT2:

$diskutil list
/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *251.0 GB   disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Macintosh HD            248.4 GB   disk0s2
   3:                 Apple_Boot Recovery HD             1.7 GB     disk0s3
$ diskutil cs list
No CoreStorage logical volume groups found

Answer 1

In order to change the size of a partition, the partition can't have any errors.

The easiest way to repair the partition is to reboot while holding Cmd+R, and start the Disk Utility from the menu that is presented after the recovery system has started.

Then select the partition and click the 'Repair Disk' button. After it has finished, it should say that there are no more errors. The partition can now be resized.

NOTE: Unfortunately, because of HFS+ it is possible that Disk Utility may display "Could not be repaired", at which point you would need to reformat the drive and restore from your latest backup. I hope this is not the case!