Is your hard drive not being recognized?
When a drive is not recognized due to defective areas on the unit, you should proceed with a software recovery.
If it is the drive hosting the operating system concerned, cautiously disconnect it and plug it into another PC in order to proceed recovery in a secured manner.
If it is the memory card of a smartphone or a tablet concerned, the best option is to purchase an adaptor and to plug it in a PC.
The hard drive: the different types of failures
SSD drives, USB sticks, and memory cards: what you need to know
The differences between mechanical hard drives and SSD drives, USB sticks, and memory cards are based on flash memory. The non-recognition could be due to poor state of the connectors. By cleaning it with the help of a cotton swab lightly dipped in alcohol, then letting it dry thoroughly, it may be possible to solve the problem.
Alternatively, it is possible that access becomes difficult, or even impossible. This is often linked to a failure in the storage unit clusters. If the SSD drive, the stick or the card is new, this could be an original defect covered by warranty. In other cases, it could be due to excessive usage and wear & tear. It is important to know that flash memory units have several writing/suppression cycles limited for each cluster, going from 1,000 to over 100,000 depending on the quality of the product. When this limit is reached, access is more difficult and it becomes a pressing matter to consider saving the data. If the drive becomes totally inaccessible, professional data recovery is necessary.
Regarding external hard drives...
Non-recognized external hard drive on a Mac
While it is often recognized for its superior reliability, MacOS sometimes cannot recognize an external hard drive. First of all, it is essential to check if the cable is OK. Also, change the USB port in case one of them happens to be faulty. If the drive is detected, but not accessible, attempt to repair the file allocation table using the system’s disk utility.
It is enough to click on "repair disk" to try to retrieve the data. If the operation fails, a dialogue box will pop up to notify you. If the file systems are not recognized, which sometimes occurs with the older versions of MacOS, do not format as you risk losing your data.