Hard drive hang on specific file
I have also seen this for mounted drives. I think it must be a re-try forever loop in the driver and it can occur on mounted drives too. It can eventually cause a cascade of other processes hanging.
The problem is previously undetected media errors. I have encountered it on USB hard disks and on freebie USB flash drives (example: a conference giveaway 1GB flash USB with a logo printed on it; since I had 2 fail on me in the past year, I stopped using free ones altogether.)
As drives capacities keep increasing, this kind of problem will happen more often. I think it is a serious bug that the driver retries forever. On a mounted drive, disconnecting the device does not break the loop (I saw that in OSX 10.6.3) and a hard shutdown is needed.
Apple was planning on adding ZFS from Sun to address this problem, but it was not included in Snow Leopard for unknown reasons. ZFS is open sourced and 5-6 years old now; it has "RAIDz" and error detection for all files, and if I list everything it has, this would sound like a commercial, so I won't... (I'm just a happy user with 10TB Opensolaris box that can handle 2 drive failures without loss.))
How to determine which file was causing the hang:
* If Finder hangs during a copy, then using Activity Monitor to inspect Finder for Open Files and Ports can help narrow it down.
* Try copying suspected files with command line "tar cvf - /path > /dev/null" (where /path is the path to the directory which has a problem file, possibly in a subdirectory). This will list files as they are read, so if a hang occurs, the problem file is the last one it outputs.
If the problem is in an OSX file, rebooting to single user mode and using the above tar command might work.