If you have an external storage drive such as those from Western Digital or LaCie, and if you have configured the drive with included software and drivers that came with the drive or which you downloaded from the manufacturer, then be aware that this software may not be compatible with OS X upgrades and could result in the inability to read the data you have on the drive.
In a growing Apple support discussion forum thread, and in CNET article comments, a number of Mac owners who have upgraded to OS X Mavericks are experiencing issues with external hard drives such as those offered by Western Digital and LaCie. These individuals have used the WD Drive Manager and similar utilities to configure the drive for Mountain Lion and prior versions of OS X, but after upgrading to Mavericks the system reports their drives as empty devices.
If this has happened to you, or if you have such a drive and are considering upgrading to Mavericks, then there are some steps you can take to ensure both compatibility and that your data is safe. First, be sure to back the data up to at least one additional location. Granted many people purchase a single massive storage device to save everything, including Time Machine backups, but this approach defeats the purpose of a backup if the device breaks or is otherwise inaccessible.
With your data backed up, check with your drive's manufacturer for any updates to the utility software you use for configuring the drive, in addition to any firmware updates that may be available for the drive. These can often be found using built-in updater services in the current versions of the utilities you have, but should also be available in the support section of the manufacturer's Web site.
Even though updates may be available that offer compatibility with the latest operating system software, be sure to double-check other users' experiences with the updates. Check the support forums for the drive manufacturer, Apple's support forums, third-party Mac community forums, and even perform Web searches to see if anyone with your specific drive make and model have had success or failure after upgrading. Determining success and failure from Web resources is a bit subjective in nature, but should give you a feel for whether or not others are experiencing problems.
Only proceed with upgrades if you have backed up the data on your drives, have ensured you have the latest software updates from the manufacturer, and if you have not found any reports of compatibility errors (or better yet, have found reports of the drive working fine for others).
If you have upgraded and are experiencing problems with your drive, then first try installing or reinstalling the latest utility software updates for the drive. Do not format or partition the drive, or set it up for use with Time Machine, even if OS X issues prompts to do so, as this will wipe potentially restorable data from the drive. With the utility software updated, attempt to use it to access and mount the drive (again, without formatting or partitioning), but if that does not work then restore to a backup of OS X -- hopefully you took our recommended preparation steps and backed up your system using Time Machine before upgrading to Mavericks.
To restore, simply reboot your system to the OS X Recovery HD partition by holding the Command-R keys immediately after hearing the boot chimes at startup. Then choose the option to restore from a Time Machine backup. Attach your Time Machine drive and choose the backup that was created immediately before you installed OS X Mavericks, then proceed with the on-screen instructions to restore your system.
When the system is running its previous operating system and software setup, you should be able to attach the drive and read data from it as you were able to before upgrading. This will allow you to access the data and back it up to another location, before again attempting to upgrade to Mavericks and configuring the drive on the new system.