Hello,
I had a similar problem using ESET NOD Antivirus Ver. 4, but I also found a (relatively) easy SOLUTION. Every time when I attached an external hard drive to the USB port, the system froze without any other alternative as switching it off. (Let's remark here that I'm using this version of ESET Antivirus on 5 different computers, and only one single computer was involved in this error, and that the issue happened with three different hard disks.) The solution of the problem was to temporarily disable antivirus functions (including real-time file system protection and antispyware protection), then plug in the hard disk into the USB port, wait until the drive has been set up and the files can be read, and then enable the antivirus functions again. Although it is annoying to do this every time, but it is quicker than performing exhaustive diagnostics to find out and fix the issue.
By the way, the external hard drive also may work if it is attached already when the computer is turned on (i.e., the virus protection is activated later than the recognition of the drive occurs).
