is it safe to unplug a USB drive while Windows is in sleep/hibernate/off mode?

The "Safely Remove Hardware" routine finishes cached/delayed writes to the drive, so data isn't left in an inconsistent state when the drive is removed. As far as I know, when your netbook changes to sleep mode it also finishes any writing to the flash drive. You can unplug the drive as soon as the netbook is actually in sleep mode (which can take several seconds after you close the lid).

If the netbook is hibernated, it should be safe to unplug the drive. A poorly-written program may throw errors when you resume if it expects the drive to be available.

If the netbook is off, it's always safe to just unplug the drive.

It might not generate any problem disconnecting drive while in sleep state.
But you have to make sure that none of the program is being reading any file from that drive(Like playing multimedia file on VLC and putting computer on sleep.)otherwise it may throw a problem on reading the file.

In hibernating the computer will force application to save changes to the file on which they are working so it`s safe.

While turning off computer is the safest way, because

1. OS will close all running application. (If application is running then it forces to close the app, force close dialog will open.)
2. If application have modified any file and file is unsaved then force close dialog will be shown. Here you get the scope to save the file, if you don`t then the application will be force closed. The file it was modifying will be on its previous state and will not carry any modification.
3. Finally it releases all the resources used by the application, and clears the memory buffer.