So, to recap, connecting the Captivate to my Windows 7 laptop via USB easily mounted it as a drive, and permitted browsing folders, creating new folders, and copying files.
However, kept getting 'USB device not recognized' on my XP desktop. Uninstalled and reinstalled all relevant drivers, but no change.
One commenter in one thread somewhere out there suggested changing USB cables. I knew the cable worked OK on the laptop, so it definitely was OK. Another guy said to try a different USB port.
'What difference could that make?', I asked myself.
Plugged the computer end of the USB cable into a front panel port, and everything worked great, soon bringing up a normal window onto the phone's file system. If you have a rational explanation for this, I'd love to hear it.
This concludes the Quest for New Phone saga. Here's a recap:
Step 1 - issues with my old WM 6.5 phone
Step 2 - let's try a Windows 7 phone - oops!
Step 3 - maybe Android will come to the rescue
Step 4 - Captivate is promising, but major setbacks cause anxiety
Step 5 - maybe everything will be OK, after all
This post - everything may actually be OK, after all.