Thanks for your answer.
In this Dell PowerEdge R410 Win2008 R2 server I have 4 disks: C: D: F: and G: According to Disk Management, C: D: F: are 3 partitions of Disk 0, and G: is alone in Disk 1. I also have E: as a CD Rom drive.
So I assumed \Device\Harddisk1\DR1 was the USB Buffalo external drive G: on Disk 1.
I just did a complete Check Disk in Disk Management (CHKDSK) for this Disk 1, no issue reported, no bad sectors, no file system errors.
In Device Manager, I see two disks, but no option there to check for issues. The driver for this Buffalo HD-WLU3/R1 external drive is "Driver Microsoft 6.1.7600.16385 (2006-06-21)". I tried to update the driver but there is no update available.
I have not noticed Event ID 11 errors since December 22, but this doesn't mean anything (Event ID 11 was reported 4 times this month on 6, 11, 16, 22 December, nothing in November, some more in October, but much less before, one in January only, not sure before).
Any more things I could check?