Actually the best thing to do is to make a complete separate verified backup of the data.
Then try to identify which drive is failing.
Try to run a check on your disk to see if everything is fine. Usually you may get your hard disk tool from the Manufacturer like Seagate.
If everything is fine, something if you put back the repaired drive, the data will still be here.
Now regarding your question about replacing the drive, yes, you will lose the data as you will need to recreate a new raid 0 with both drives and the raid array will be auto-initialised thus deleting all the data.
Hope this helps a bit.
All the best,