Try a 4GB or smaller USB drive. I couldn't get mine to work until I used the smaller drive.
Not to beat a dead horse, but...
We can add a re-capped ZX Spectrum +2 to the list of my computers that have taken a bath.
And I have modified my procedure, I now rinse my board in 91% IPA to dry it out (the IPA absorbs the water left over after rinsing the vinegar off) before using my hot-air rework station to dry it off the rest of the way. Much quicker.
He's a picture of it after the re-cap, sound mod installation, and the removal of the UHF modulator (I used the hole for the video connector for the audio jack) as well as a shot of the back of the board. It really does look brand new.
And to be fair, there wasn't any leaky caps (that I saw anyways) when I replaced them. So there wasn't any trace damage that I could observe.