I get this same error message when I try to eject my external device from the LEFT column (the Navigation Pane) of Windows Explorer, but I don't get the error message when I try to eject it from the RIGHT column. See if that makes a difference for you.
My Computer
Computer Manufacturer/Model Number
HP Pavillion dv5t (generation 1)
OS
Vista 64 bit and 32 bit (SP2)
CPU
Intel(R) Core(TM)2 Duo Processor T9400 (2.53 GHz
Memory
4GB DDR2 System Memory (2 Dimm)
Graphics Card(s)
512 MB NVIDIA GeForce 9600M GT
Monitor(s) Displays
15.4" diagonal WSXGA+ High-Definition HP BrightView Widescre
Screen Resolution
1680 x 1050
Hard Drives
320GB 5400RPM SATA Hard Drive with HP ProtectSmart Hard Drive Protection
Keyboard
Built-in HP
Mouse
Built in - Synaptics TouchPad V6.5 on PS/2 Port
Internet Speed
Max
Other Info
~ Intel Next-Gen Wireless-N Mini-card w/Bluetooth ~ Blu-Ray ROM DVD+/-R/RW ~ Integ. HDTV Hybrid Tuner ~ 12 Cell Battery ~ MS Office (Home Premium) 2007 ~
What are you using the external HDD for, backups? If so, try turning off the windows backup service or what ever backup service your are using and then see if you can eject HDD.
My Computer
Computer Manufacturer/Model Number
Custom built
OS
Windows 7 Ultimate 64 bit
CPU
AMD Phenom II X4 965 Black Edition
Motherboard
Gigabyte GA-MA790X-DS4
Memory
GSkill 4 X 2 GB PC 8500
Graphics Card(s)
XFX Radeon HD 6790 D
Sound Card
On board RealTek HD
Monitor(s) Displays
Dual monitors:Samsung SyncMaster S20B300
Screen Resolution
1600 X 900
Hard Drives
Seagate Barracuda 1TB (primary)
Seagate Barracuda 2 X 320 GB
PSU
Ultra X4 750 watt fully modular
Case
Thermaltake Overseer RX 1 full tower
Cooling
Core-Contact 92 mm CPU Cooler
Keyboard
Logitech G510
Mouse
Razor DeathAdder
Internet Speed
50/5 Mbps UL/DL
Other Info
Optical: Super Muliti DVD burner w/lightscribe, Hauppauge WinTV HVR-1800