If a hard disk fell or was subjected to physical impact, do not connect the power cable to it.
If disk emits strange sounds, skirrs, clicks, etc., it should be immediately disconnected from power outlet.
It is not recommended to perform testing of failed hard disk drives with the help of software.
Memory cards and flash cards:
If USB flash or memory card is seen as a zero-sized drive or as an unknown device, then, most probably drive's controller circuit has failed. Internet has a lot of tools and software to mend this problem. But be careful because in 90% of cases when such methods were applied the recovery is not possible. And if data is really important, then you should not apply this procedure to your hard disk drive.
RAID arrays of all levels:
In case one of the array disks fails and it was subsequently replaced with another one, you should not interrupt array's rebuild procedure. Also, during the array rebuild procedure you should not swap positions of hard disk drives.
If you use RAID 1 array (mirroring) or its analogues, be careful and follow disks' functionality and timely replace failed hard disk drives.
Date recovery process unfortunately is not some magic trick, to the contrary, it is a consistent, technically difficult process and its success depends a lot on the drive's condition and actions performed on and with it.