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The flash drive appears on your Mac computer’s desktop as soon as you connect it to the USB port, these devices also appear under Devices column on the left side Finder to ensure that they are easy to find and locate.

However, at times these external devices do not show up at all, thus making it difficult for you to access the stored data. This situation can be annoying, especially when you need to access something important. When such a thing happens, it doesn’t only mean that you are out of bounds to access data, but also there can be a chance that the storage device that you wish to access, such as USB, flash drive, or hard disk is corrupted.

Reasons for ‘Flash Drive Not Showing Up Mac’ Error
Before we proceed to fix and repair the error, Mac users can understand the most common reasons or situations that can cause the ‘flash drive not showing up mac’ error on your Mac system. Therefore, please refer below to learn some easy ways to fix the error according to the different situations.

Situation 1. Flash Drive is Not Plugged Properly
In this case, please check if the storage device is connected properly or not to your Mac device. If not, connect it properly!

Situation 2. Damaged Storage Device
Try to reconnect the storage device to a different Mac device to check whether it is in good condition or not. The “flash drive not showing up mac” error can also occur when the device is physically damaged.

Situation 3. Faulty Cables
Sometimes error occurred because the cable that you are using to connect the device is loose or wobbly. If so, please try to use another cable.

Situation 4. Incorrect Drive File System
Not every drive is compatible with Mac, so it could be that you are trying to connect the drive that is not meant for the Mac system. This can be checked by navigating to Apple, at the top Toolbar Menu, and then go to About This Mac, followed by Storage. Please check if the flash drive Appears in the storage or not.

Refer here to fix Flash Drive Not Showing Up Mac with Data recovery software for Mac.
:) :)