Re: restart power off time
Some Ext. USB HDD do take up time to poweroff. Usually this happens if some application or another is using it and/or if data is being written to it. If its a NTFS file system them perhaps you can run 'defragmentation'.
Does this Ext.HDD have an OS on it?
If its sending you to the 'grub rescue' then your BIOS settings are perhaps set to boot USB devices first.... you can fix it by changing the HDD boot order in the BIOS to boot your OS/Grub installed HDD first.
"Evolution is Nature's way of issuing upgrades."