There is currently a nasty bug in Fedora 17 related to USB devices. If you copy some data to your USB flash drive/harddisk and try to eject it (either in Nautilus or using Gnome Shell tray icon), there is no indicator of progress saying whether you need to wait until all the data are written or whether you can already remove the drive. Instead you’ll get the notion that you can remove the drive immediately. But if you do this, your data might be corrupted (in case some of them haven’t been transferred yet).
There are two possible workarounds for this:
1. (easier) After you eject the device using Nautilus or Gnome Shell dialog, run a terminal and write sync. Wait until that finishes and only after that unplug the device from the computer.
2. (harder) Don’t use Nautilus or Gnome Shell to eject the device, but use umount in terminal instead. You have to be root to do this. The mount point is in /run/media/.
As you can see, none of these approaches is suitable for inexperienced users. I’ll try to make sure we won’t release Fedora 17 Final with this bug included. Until that time, please be aware of this so that you don’t lose your data.
EDIT: Follow-up post: Fedora 17: Your data on USB devices are now safe(r)