Computers do not "recognize" drives (memory cards is essentially a thumb drive) due to formatting (MBD) issues.
That neither a PC or a Mac can read the format could be caused by a camera writing a proprietary format, or the card being damaged. The only other possibility for why a Mac could not read a format is that it is formatted NTFS by Windows older Mac's can not read this format - new Mac's can read- but not write to this format. Never erase or format a camera card on a computer that must be accomplished in the camera only, by the camera's software/OS or your risk making the card unusable by the camera.
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