Windows 7: Problem Copying to eSATA Drive

I was running Windows 7 Ultimate.  I was copying files from an internal SATA hard drive to an external SATA drive via eSATA cable.  The external unit consisted of an internal drive inside a Rosewill RX-358-S SLV external enclosure.  I was copying files just fine, and then suddenly I got this error message:

Problem Copying

An error has occurred.

The destination you have specified does not exist.  It might be an offline network location or an empty CD or DVD drive.  Check the location and try again.

I noticed that the in-use light on the external enclosure was now constantly lit.  I also saw that the external drive was now missing from the folders pane in Windows Explorer.  I had done similar copying operations in recent days.  This was the first time I had had this problem on this new computer installation.

I did a search for part of that error message.  One thread contained a bunch of suggestions that did not apply to my situation, and so did another.  But I did notice a suggestion that looked like it might help.  To apply that suggestion, I had to get the drive back.  To do that, I rebooted and hoped that Windows Explorer would see it once again.  It did.  So the suggestion, in Vista terms, was to right-click on the drive in My Computer (or in the Windows Explorer folders pane) and go into Properties > Hardware tab > select the drive > Properties > Policies tab > select the "Optimized for performance" option.  I didn't have that option.  There was a write-caching option, but that didn't seem to explain the sudden disappearance of an entire drive.  I could have set Control Panel > System > Advanced system settings > Advanced tab for best performance of background services, but (a) I didn't want to change the whole system for the sake of this one task and (b) I didn't want to prioritize background services.

I tried a different search.  This Rosewill unit had both eSATA and USB connectors.  I was using eSATA because it was much faster.  It had the drawback of requiring a reboot to be recognized.  One poster indicated that s/he had the same problem as I, but only when using his/her unit's eSATA rather than its USB connection.  Otherwise, I was not finding a solution.  I restarted the copying process, still using the eSATA port.  The problem did not recur that day.  I decided to wrap up this post and follow it with another if the problem recurred.