After upgrading to osx 10.9.3, my external hard drive was not able to mount, but it can be mounted without issue in osx 10.6, how do I solve it? it was formatted by disk utility.

Back up all data to at least two different storage devices, if you haven't already done so. There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

Erase and install OS X. This operation will destroy all data on the startup volume, so you had be better be sure of your backups. If your Mac was upgraded from an older version of OS X, you may need the Apple ID and password you used.

If you use FileVault 2, then in Disk Utility select the icon of the FileVault volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another icon with the same name. Click the Unlock button in the toolbar and enter your login password when prompted.

When you restart the computer, the Setup Assistant will launch to take you through the initial setup process. Follow the prompt to transfer the data from one of your backups. Don't transfer the Guest account, if it was enabled on the old system.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode.

Just to be clear, I'm following this correctly. Your Seagate GoFlex drive used for backup no longer mounts after upgrading to OS X 10.9.3.

If the drive is formatted with the Seagate software, that could be the problem. It's best to format with Apple's Disk Utility rather than the manufacturer's software. I have Seagate drives and have no problems. I erased and formatted with Disk Utility when purchased.

If possible copy any data off the drive on a different Mac then erase and reformat.

do i need to go through to install my other applications like virtual box, window 8, MS office 2011, etc?

If you follow the instructions, they'll be restored from a backup.

If you boot into the Recovery Drive, does Disk Utility see the drive?

If yes, select Repair Drive.

If no, copy the data off the drive on the old computer then attach drive to new Mac and format the drive. Copy some test data to the drive to verify it works. Attach drive to source where you backed up your data and copy to the newly formatted drive.

Boot into the Recovery Drive by holding down Command R when restarting.

I have been able to resolve this for a friend's iMAC this was running 10.9.3 with the same issue external HDD or USB thumb drives would not mount but were visible within Disk Utility.

The approach taken:

Upgrade to 10.9.4

Run repair permissions on the Mac HD

If Tuxera NTFS is installed, try to uninstall and remove from System Preferences

(note that this may hang)

Reinstall Tuxera NTFS

system will restart

HDD will now mount correctly

No reformatting required

good luck