Dear Esam,
Activesync is a very stable program. However it is very dangerous too. It can synchronize the contents for both your PPC and PC; however after you establish a partnership it stays. Personally I don't think there is any problem with your SanDisk card, neither your Activesync program. Hence I would like to share some tips that you might find useful (These are my personal preferences and not rules!:

1. You have to decide whether the PC or PPC is your main computer for your main use.
2. Establish Standard partnership only when you want to physically synchronize data.
3. If you are installing programs or copying files, establishing a guest partnership is more than enough. (Which in my opinion is 90% why I place my PPC on the cradle)

4. Always backup your data. For contacts and appointments, back up them in both your SD and Storage folder (as the contents of the Storage folder stays even if a hard reset is made.

For full device data, use the backup/restore option in the ActiveSync tools menu for further save on a re-writable CD.
I have to admit, that "Activesync" needs more improvements to prevent accidental synchronization as soon as a standard partnership is ticked. More anticipation descriptive dialog boxes and confirmation questions should appear before any synchronization takes place. Hopefully Microsoft will modify this feature in future releases.