\"Failed opcode was: 0xef\" considered harmless

This morning I noticed these entries in the logs of my monitoring machine at work:

hda: drive_cmd: status=0x51 { DriveReady SeekComplete Error }
hda: drive_cmd: error=0x04 { DriveStatusError }
ide: failed opcode was: 0xef
hda: task_no_data_intr: status=0x51 { DriveReady SeekComplete Error }
hda: task_no_data_intr: error=0x04 { DriveStatusError }
ide: failed opcode was: 0xef

After a lot of Googling, I managed to find a few things that explained it:

• This bugzilla comment from Dave Jones explains "51/04 is the drive saying I don't know what that command means after being told to do something. It's not an error." The failed opcode — in this case, 0xef — is the operation that's being tried and confusing the HD.
• This page says "yep, that'll happen, all right" (no, not a direct quote) with SLES 9 and Seagate drives, and says you can ignore it. No explanation why.
• Finally, this post to the LKML says that opcode 0xef is telling the hard drive to keep its settings over a reset, and can be produced using hdparm -K 1 /dev/hda. Sure enough, running this produced the error in the logs, and this one on the screen:

setting drive keep features to 1 (on)
HDIO_DRIVE_CMD(keepsettings) failed: Input/output error

This is a completely benign error, of course…I really don't care if we have to run hdparm with every boot. I had also tested the drive by booting into Knoppix and md5summing every file on the drive — no errors produced at all.

Don't know what's worse — wasting two hours on this, or not noticing it before now. At any rate, this failed opcode appears to be completely harmless.