I've reformatted the card several times and am still having the same problem. I've also tried another sd card.
There's a reported bug in the gallery app. in Sense 3.0/3.5 that seems to be the culprit. It's not popping up in ICS, yet, so hopefully it stays that way. HTC never provided a fix, not for lack of trying. You have to try all the standard fixes and hope one gets you back on track, though nothing is guaranteed. Since the reformatting didn't work out. Here's few more, in no particular order:
I. Factory reset from the hboot menu (pain in butt, should fix the issue, though)
II.. Unmount/Remount SD ext_SD, reboot
II. Download SdRescan in the market, run it, see if it helps
III. Download a third party gallery app like QuickPic
IV. Clear out HTC apps cache, in settings
V. Should scan it on every boot up. Never tried it, so who knows, pull out the sd card entirely or exchange the SD, reboot, let it load completely, power down, replace the SD and reboot. That would force the card reader to scan it anew.
VI. Check fast boot in settings, if checked, I believe I once read it omits sd scanning new files on boot up (for non rooted, diff. purpose rooted)
VII. Maybe try backing up your SD card content, clear out the gallerys thumbnail cache entirely, reboot
Good luck