Meaning of "data underrun"?

Thanks.

This is my Time Machine disk. As far as I can tell, I've seen the errors have been occuring just before a backup starts. I don't think the drive ever gets spun down, though, so it can't be a spin-up issue. (The maker of the enclosure suggests either the drive or the enclosure can cause this. I will test this.)

As to whether there is data loss, that really depends on if the underrun is detected by an error and whether the filesystem/driver layer retries it. So the lack of an obvious error message in the log worries me somewhat.

I have tested the latest backup for integrity with a lot of diffs and there are no indications of corruption, so far. I did find a few files from the 10.7.4 update which were not getting backed up for some reason.

I was unclear in my original post. The message was "disk1s2:data underrun", so I know it isn't the internal. I've swapped another disk into the enclosure and it hasn't gotten an underrun so far, but I need to run it a few days to be sure.

As to the Time Machine problem, I'm assuming I did something that caused it to miss a bunch of fsevents. Time machine needs a "scan the entire disk" option and a real verify backup option to handle certain kinds of problems.