sync doesn't terminate after cloning disk using dd

I ran the following command to clone the hard disk (128 GB) on my computer (running Ubuntu 14.04.5, kernel version 3.13.0-110 on 64 bit x86) to a partition on an external drive:

dd if=/dev/sda of=/dev/sdc1 bs=32M conv=sync,noerror

Note that /dev/sda was mounted when I ran dd, which (dd) terminated after an hour or so. I then ran sync which I normally do before removing external hard drives.

However, 6 hours later, sync hasn't terminated.

The 'Dirty' number when I run grep -e Dirty: -e Writeback: /proc/meminfo keeps hovering between 0 and 200Kb. It doesn't monotonically decrease as suggested here: https://unix.stackexchange.com/questions/48235/can-i-watch-the-progress-of-a-sync-operation

When I run the iostat command, kB_read/s and kB_wrtn/s numbers are both constantly changing for both sda (local hard disk) and sdc (external hard drive). kB_read and kB_wrtn numbers are only changing with respect to sda. They're constant for sdc.

Device:            tps    kB_read/s    kB_wrtn/s    kB_read    kB_wrtn
sda              26.70      2423.42        18.41  128455820     975976
sdc              46.09       444.73      2782.10   23573587  147468196

Can I just yank out the external USB drive (sdc)?

Update, thought I should add the output of fdisk -l here. Note that I'm copying an entire disk with 3 partitions to a partition on another disk (not sure if there's anything wrong with that):

Disk /dev/sda: 128.0 GB, 128035676160 bytes
255 heads, 63 sectors/track, 15566 cylinders, total 250069680 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0000138b

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048   242845695   121421824   83  Linux
/dev/sda2       242847742   250068991     3610625    5  Extended
/dev/sda5       242847744   250068991     3610624   82  Linux swap / Solaris

Disk /dev/sdc: 500.1 GB, 500107861504 bytes
255 heads, 63 sectors/track, 60801 cylinders, total 976773167 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x000b72e0

   Device Boot      Start         End      Blocks   Id  System
/dev/sdc1   *        2048   263731199   131864576   83  Linux
/dev/sdc2       263731200   970629119   353448960   83  Linux
/dev/sdc3       970629120   976773119     3072000    7  HPFS/NTFS/exFAT