The long-awaited Sprint HTC EVO 4G was released this morning and met with long lines at Radio Shack, Best Buy, and other resellers.

Just days ago we learned of a problem where the device could not reliably access the pre-packaged microsd card. Many users reported files “disappearing” or were met with an “insufficient file permissions” error when attempting to save files to the card. The work-around was simply to reboot the device, access to the files would be restored and files could again be written to the card — for a while. Others reported that reformatting their sdcard on a desktop computer resolved the issue.

HTC has identified and corrected the root cause of the problem (but did not go into detail about the cause) and has released an over-the-air (OTA) update that will launch when the device is first powered on.

The update is approximately 13MB and takes around five minutes to download and install.

To check and see if you have any updates available on your EVO, go to Settings, System updates, HTC software update, Check now.

On a related note, this patch also plugs a “security hole” through which users could obtain root access to the device, meaning we’ll need to wait for another route-to-root to be discovered if you want root access on your EVO.

Two future updates are expected: a radio update to improve performance and battery life, and Android 2.2 (“Froyo”). Timelines for these updates have not yet been announced.