Thank you for both replies. My apologies for the very long delay for this update.
There was apparently nothing wrong with the SanDisk USB 3.0 UFD.
The problem was Windows 8.
Windows 8 would detect the USB 3.0 UFD the first time it was inserted in a USB 3.0 port, but not subsequently. This problem affected both USB 3.0 ports on the computer. It also occurred with two identical SanDisk USB 3.0 UFDs, one whose factory partition was deleted and then undeleted (as reported in my original post), and a second new UFD that was inserted into the USB 3.0 ports straight from its bubble pack.
After the initial detection, Windows 8 would only detect the UFD if either (1) it was inserted in a USB 3.0 port before Windows 8 was opened, or (2) inserted first in a USB 2.0 port, removed, then inserted in a USB 3.0 port.
However, although the UFD was in a physical USB 3.0 port, when detected, Windows 8 assigned it to a logical USB 2.0 port. The reported connection was USB 2.1, which I believe means a USB 3.0 device in a USB 2.0 port. The port assignment and connection were found using USB Device Tree Viewer 2.5.8 and USBDeview 2.46, both freeware programs.
Neither Windows 8 built-in "Hardware and Devices" troubleshooter nor the Microsoft Fix it for "Hardware devices are not working or are not detected in Windows" (https://support.microsoft.com/en-us/mat ... e_problems
) found any problems.
Used Windows 8 Device Manager to check for updates for the Windows 8 built-in Intel USB 3.0 eXtensible Host Controller and USB Root Hub (xHCI) drivers, but it reported that the latest drivers were already installed.
Found no solution on Internet, except for a website that reported Windows 8's built-in USB 3.0 drivers did not work with some USB 3.0 devices, and recommended replacing them with drivers designed for Windows 7. (Intel never released USB 3.0 drivers designed for Windows 8.) This recommendation was for Windows 8 only, and specifically not for 8.1.
Before doing this replacement, Windows 8 suddenly and inexplicably began to correctly recognize the UFD in a physical USB 3.0 port, i.e., the UFD was assigned to a logical USB 3.0 port, and the connection was USB 3.0, not 2.1. Moreover, the UFD could be inserted at any time, so no need to do so before Windows opened, or put it in a USB 2.0 port first.
I don't know what caused the problem, or how it fixed itself, but it hasn't reappeared since.