Image and video files disappear from the SD Card Sometimes it happens that all image files (JPEG, PNG, etc.) and videos (3GP) are erased automatically in the SD Card. So they are removed from Gallery also.
This means I risk to lose documents and photos I shoot with the phone camera, unless I make external backups.

I noticed that sometimes it looks related to fast mount/unmount cycles via USB cable to a PC (due to unwanted multiple pressure on the 'Activate/Deactivate media archive' soft-button on the phone)

Does anyone know what happens? Is it a known bug, some misuse from my side, etc.

My phone model is LG P962, with Android 2.3.4 and a 32-GB SD Card.

Thanks!
I would rather go with replacing the SD Card and checking if that happens again. Nowadays there are lots of cheap SD Cards which simply lose or corrupt your data with no particular reason. There have been reports of SD cards that pretend to be 4, 8, 16GB or even more but only have 256MB. If you have such a bad card and copy more than 256mb of data it will get corrupted. The same thing applies to flash drives etc.

If that doesn't help then try to restore your device to the original state and do not install all your applications at one go. Try to install the most important first and the rest in some intervals so you can find the one which may be causing problems.