Description of problem:
Error occurs when detach a disk on a running VM(rhel6.7). This disk is mounted to a mount point.
After detached, it's still in the mount list but removed from device list.
Same on Ubuntu. But could not reproduce on rhel7.2.
Meanwhile, on Azure, the disk will be detached even it is busy. But on hyper-v,
the disk will be detached only when it is not in use.

Version-Release number of selected component (if applicable):
WALinuxAgent-2.0.16-1.el6

How reproducible:
Always

Steps to Reproduce:
1. Attach a disk to a running VM.
2. Mount the disk.
# mount /dev/sdc1 /mnt/newdisk
3. Detach the disk on the azure portal.
4. Check the disk on VM
# mount  | grep /dev/sdc1
/dev/sdc1 on /mnt/newdisk1 type ext4 (rw)
# fdisk -l | grep sdc
#
# ls /mnt/newdisk
ls: reading directory /mnt/newdisk1: Input/output error
#

Actual results:
After detached, it's still in the mount list but removed from device list. And it returns an error
when do any manipulation on the mount point.

Expected results:
After detached, it shouldn't be in the mount list any more.
on Azure, the disk should not be detached when it is busy.

Additional info: