USB removal bug?

Today I encountered a strange bug (i am not quite sure whether it is a bug!). When my friend’s USB was put in my computer, for taking a print out, CIS promptly found a virus (autoit worm), which I quarantined immediately. After taking print out, however, my computer did not permit me to remove the USB disk with ‘cannot stop the volume right now - generic volume message’.

Thanks to Comodo the virus is removed and I deleted it from quarantine and also removed the system restore to stop its returning. However, I fail to understand why it could not stop the generic volume.

In CAVS 2.0 I remember that this was a bug, which was removed in CIS. Hence, to check whether this is a bug, after I removed his USB, I checked it with my own and there was no problem removing the USB. So, is there any known bug that in case a virus is detected in a USB, the antivirus lock the drive? I hope this is not a bug and was just a one off case. If it is a bug, the devs may please check it.

BTW before stopping the USB, I actually stopped the .doc file, no printing / spooling work was pending and I also closed the windows explorer twice, to ensure that the programs are not using the USB disk. It was also ensured that no programs are running (in applications) in the task manager.

Windows XP Professional SP2
Only CIS (no other security applications)
Try to reproduce failed as explained above
(seems that it happened just because a virus was detected in that USB).
I was able to remove this same USB two days before, when virus was not detected or present in the USB.

Over to experts.

Confirmed the above bug. Virus detected by CIS

worm.win32.autoit.DB@470708

USB could not be removed (i mean stoppage of service and safe removal through right click option) even after it is removed by CIS.

However, if I manually remove the USB instead of stopping the service and reinsert the same USB, I can stop the service and safely remove the USB.

Thanks

In earlier versions, Comodo antivirus did not allow removing USB devices at all. It looks like this bug is still present in some cases even now.