Try this update from microsoft

Look here and download

Do you have any backup or imaging software installed which uses your external drive as a destination for backups? Some of these applications have services running which "trick" Windows into thinking that the external drive is in use. Have a look at the running services and see if any of these might give a clue. On a few of my systems, I have to stop a particular service, then "safely remove hardware" and then start the service again.

After down load and installed the windows update, it became sometime work and sometime not work but better then before. It not work at all before!

Thank

-Inspiron 8500:
P4 2.5Ghz
1GB DDR RAM
60gig Ultra ATA HD
64MB NVIDIA GeForce FX Go4200
100Mb Ethernet
15.4" Wide Aspect Ultrasharp WUXGA

-Dimension E520
Core 2 Duo E6400 2.13 Ghz
256MB NVIDIA GeForce(R) 7300LE
250GB SATA 3.0Gb/s Hard Drive
2GB NECC Dual Channel DDR2 533MHz SDRAM
22" Entry Wide Screen Flat Panel LCD Monitor
BIOS DM061 2.4.0

Glad to hear that

And try defragment, check for errors too, it will be better 

Good Luck 

Hi - can you tell me if I need this update to solve this problem if I have the vista sp1 update? In other words is this included on vista sp1 macines already?

Thanks

I don't no its included in sp1 or not

But I 'm using sp1 and downloaded this update too, every thing goes fine, no problem

But I'm using program called Unlocker, its good for USB problems

You can download it from here

Good Luck

Thanks for your reply. I used unblocker and it doesn't show any active processes. I have done a restart and its stopping ok for the moment. If it starts again I will download the update and try it. Best wishes