magic wand corrupts drawing

Most likely the file is fine.

If you save the selection to an alpha channel, Windows Explorer/desktop will interpret that as transparency and display it accordingly.

In a lot of applications, an alpha channel atomatically equals transparency, although that is not necessarily correct (according to the file format specifications of the formats that support alpha).

Can you provide more information about the file format? Do you have Bridge installed? If possible, please preview the image using Bridge and share some screenshots. We are not familiar with what you see on the screen when making a selection, nor what occurs after the selection is completed.

The Magic Wand is not always 100% accurate. You could be selecting more or less than it appears. Check the Tolerance setting for the Magic Wand.

Do you do anything after using the Magic wand?

Are you viewing at 100% zoom ratio?

Can you show us a screen shot of your entire workspace?  It needs to be full resolution, and show the image, and layers panel with the affected layer selected.