I/O Error on USB attached to NAS

This has worked flawlessly until this past week.

Now I am getting the "Cannot copy ...: The request could not be performed because of an I/O device error." message.

It is a Seagate 2TB USB 2 drive connected to a ReadyNAS Ultra 4 Plus, firmware 4.2.22. My network is managed by an ASUS RT-N65U router and multiple D-Link switches.

There have been a boatload of Windows XP updates lately, so I was thinking it could be that. Also NOD32 4.2.71.2 is running on that system. In the past, AV updates have sometimes caused havoc.

I thought it could be a router issue, and discovered there was a firmware update for it, so I did that. After some reboots of all systems and devices, I was able to get it to work.

Now the issue is back.

My Mac system also had no problems with the drive until recently, and it too complains about write access. Read is fine. Right now it won't delete the folder I was trying to copy in to because it says it isn't empty. Windows says the same thing. Seems empty!

I also plugged the USB drive directly into the Windows computer and ran a Seagate test. The drive tested fine.

What would be contributing to this intermittent error message, and how might I go about finding the cause?

Thank you.