USB SanDisk 4GB

Unable to detect the disk drive with a message that “Disk is not formatted”
Appreciated someone could reply me as soonest as possible. TQ

How old is the USB stick?  What OS is running on the pc your pluging it into?  Does it work on other pcs?

almost 3 yrs, XP SP2, did try to pluging into notebook running with vista… same message appear. TQ

The drive could be worn out or the FAT could be screwed up if the drive wasn’t safely removed the last time it was used.  You should try formating it, preferably in an XP machine, and if that doesn’t work you’ll need to look for a replacement. 

Is the data inside will be erased after formatting?

yes, formatting will erase the data. 

However it may be possible to recover some of the files after reformating with utilities such as this one:

And there are others which are fee based.