I managed to solve the issue by booting a GParted live CD/USB and following the accepted answer on this page: http://askubuntu.com/questions…ith-paramters-on-live-usb
(I initially tried these steps: http://gparted.org/display-doc…ted-fix-grub-boot-problem
but received an error when running grub-install, I suspect because --root-directory was not provided)
I'm very happy to have an extra SATA port now 
I am experiencing the same issue. I am attempting to install OMV (1.9 amd64 ISO from sourceforge) onto a USB hard drive (in order to save a SATA port) from a bootable USB thumb drive.
The installation goes off without a hitch... except when GRUB installs. I noticed that the USB thumb drive (OMV ISO) is identified as /dev/sda and my USB HDD is identified as /dev/sdb. During installation I get a warning about multiple disks present. I select the USB HDD (/dev/sdb) and the installation proceeds. When it gets to the GRUB installation, I noticed (only a brief moment on-screen) that it was installing GRUB to /dev/sda (my USB stick), and sure enough after installation I cannot boot from the usb stick anymore and have to re-image the ISO (with Win32DiskImager). I also could not boot the USB HDD that OMV was installed on.
I tried tx.canyon's solution:
~#: grub-installer /dev/sdb
but I get the error: "ls: /dev/sdb/proc: Not a directory"
As part of another test, I tried a freshly imaged USB drive onto the same HDD (except now plugged into a SATA port) and the installation was 100% successful.
Has anyone else managed to install OMV onto a USB hard drive?
I've used OMV in a VM as well as inside ESXi 5.5 -though I didn't have the problem you're describing. Why not just log into the console through ESXi and run the 'sudo poweroff' command?