. I now believe that the procedure described here wasn't actually what solved my problem. Sorry. See the detailed update
(I refuse, here and elsewhere, to use the term 'Mac OS X 10.4.10', so I switch occasionally between Mac OS 10.4.10, which grates because it omits the familiar X, and Mac OS X.4.10, which grates because it's hideous and internally inconsistent.) Well, as has been pointed out elsewhere, it's unexpected -- 'unprecedented', I suppose, is strictly true but a bit overdramatic -- to get an update to X.4.10 in place of the release of X.5.0 (or does it start at X.5.1?), but that's not about what I wish to post.
I have long had a problem formatting USB drives on Macs -- I filed Bug #4670521 about it on bugreport.apple.com (I'm not sure how, or whether it's possible, to link to bug reports) -- and so, when I saw that X.4.10 was supposed to offer improved reliability for USB drives, I got excited 1 -- so excited that I decided to upgrade immediately, rather than waiting the week or so I usually do to see what consequences shake out 2. What do you know! -- immediately upon applying the update, even before rebooting, my USB thumb drive, which had previously worked like a dream 4, immediately ceased working. I normally don't reboot immediately after an update, because I'm running a Subversion server on my computer for a few papers I'm writing and don't like to have unannounced downtime; but, in this case, I really needed the files on that thumb drive (for example, it contains my Firefox profile), so I rebooted (sorry to any of JA, CC, JG, and JL who were affected by it) -- twice, because the updater wants to rereboot -- and hoped for the best. I inserted the drive and -- the suspense is so thick that you can cut it with a knife 5 -- nothing. I removed it, tried it in another USB slot, and -- nothing. In fact, now the little power indicator on the drive wasn't even illuminating.
Yee-ha! Taste the improved reliability!
I have no idea what was causing the trouble, but, by deleting my kernel extensions caches (see Common workarounds for when things go wrong post-update in the linked article), I seem to have recovered my beloved thumb drive.
UPDATE 1 July 2007. Sorry to any folks who are coming here to try to find out what to do about USB drive issues -- that MacFixIt article appears to have gone behind the subscriber wall. The fix that worked for me was to delete the following:
com.apple.kernelcaches (a folder in /System/Library/Caches)
Extensions.kextcache (a file in /System/Library)
Extensions.mkext (a file in /System/Library/)
and then restart.
UPDATE 3 July 2007. It seems from the comments on this MacOSXHints story that I just got lucky with deleting the kernel extensions caches. Some commenters there guessed that the Mac OS X.4.10, v 1.1 update will fix this problem, but I can't tell: Despite instructions on the download page telling me it's recommended for all PowerPC and Intel users, I can only find the download page for the Intel version. Oh, well. Any other suggestions in the comments?
By the way, according to DeepApple (or at least to Google's automated translation of it), the problem a lot of people are having is that the drive is showing up in Disk Utility, but refusing to mount. My drive wasn't even showing up in Disk Utility, so it may be a separate problem entirely.
UPDATE 14 March 2008. I've noticed in the course of later updates that any thumb drive which is connected to the computer when it boots will not be mounted (or, at least, I can't figure out how to make it mount); but another reboot (with the drive removed) seems to solve the problem. I incidentally applied such a reboot in the course of the efforts described here, I now suspect that that, not deleting the kernel extensions, was what solved my problem.
UPDATE 17 July 2007. Wow, a lot a lot of people are still interested in this issue. Hey, folks! Sit and stay a spell!
1 Yes, I am a geek to get excited about the potential to format external USB hard drives. Even worse, the bug is still listed as open, so I don't think I was even properly geekily excited. (The workaround I had involved borrowing extra equipment from several people, and testing it only to find out it's not resolved would entail borrowing all that equipment again, so I'm not going to do so.)
2 For example, I found out in this way about the bizarre 3 Eject key behaviour introduced in X.4.9, which was enough to keep me from applying the update for a while. (I caved eventually, because I figured being up-to-date on security updates trumped having to press the Eject key for slightly longer; and, though I am still bothered by the delay, the world has not, contrary to all expectations, ended.)
3 Bizarre because it's not customisable, not because no-one would ever want it.
4 Actually, this is a simile so in-apt that one might almost think I worked at making it so; for, in fact, the drive worked perfectly except if I inserted it while the computer was asleep and then woke it, in which case it wouldn't be recognised and I'd have to insert it again.
5 This is another disappointing simile, though this time, I think, the fault is not mine. Why is it supposed to be a measure of thickness that something can be cut with a knife? Indeed, knowing that something can be so cut places an upper, not a lower, bound on its thickness: "The model was so thin I could cut him/her with a knife" (but, err, maybe that's not such a great sentence).