Optimize for performance screwd up external hard drive

Hello,
I've got a problem with a Maxtor DiamondMax 10 200GB ATA.

I mounted it into a USB external enclosure. All was fine until one day I
tried and set it to "optimize for performance". I thought it was ok, as long
as I used the "safely remove hardware" procedure, but hell set off. The
drive started to behave erratically and tons of EventID 51 warnings (An
error was detected on device \Device\Harddisk1\D during a paging operation)
showed up in the event viewer.

I reverted it back to "optimize for quick removal" and actually the drive
seems ok now, but whenever I connect it to a USB port, several EventID 51
warnings keep appearing in the event viewer.

I have a twin hard drive (exactly same model) that does not show any
problem, so I think that something wrong was left behind when I set it back
to "optimize for quick removal".

Microsoft support center says that a IO_WARNING_PAGING_FAILURE is a minor
issue on a secondary hard drive, yet this is not normal. I'm wondering what
actually happens when the write caching and safe removal options are
changed. Are changes applied only to Windows or is something written onto
the disk?

How can I make sure that all settings relative to that unit are correctly
reset? I thought of deleting the registry key referring to that device under
HKLM\SYSTEM\CurrentControlSet\Enum\USBSTOR, but I don't know if it is safe
or if something else should be checked.

Thank you for suggestions,
Luke
.