Win 8.1 hangs on shutdown, ext. hard drive not dismounting

[RodimusPrime98]

I have been rebuilding my PC with a clean install of Windows 8.1 Pro x64. All updates in place and most of my programs are installed. While configuring some programs for cloud back-ups I need to change drive letters of external drives so they match the previous installation for the backup service. Most drives changed fine using the Disk Management but one 4TB external drive hung on making the change and change did not take effect. Ever since that drive hangs the reboot but if I disconnect the drive the PC reboots instantly. So I tried to run chkdsk /f /r /x from both an elevated cmd and from the recovery cmd and I get an error that the drive cannot be locked. Diskpart shows the disk and the partition on the drive as read-only = no. I believe something must be corrupt on the drive and not allowing it to dismount and lock the drive for scanning. I am completely stumped. The drive is a Seagate Backup Plus variant.