EXTERNAL HD NOT MOUNTING

Ok - late there early here - Canada.

Move the proem external drive to the Windows computer. Confirm it functions as expected. Move ALL Data Off the drive to a Safe and Reliable location for Safe Keeping. The Data can be move be back later.

With the drive attached - format the drive to Fat 32 - what the drive to be read / writeable when moved back to Apple Computer.

Move the drive to the Apple Computer and confirm it is mountable.

If so - open Disk Utilities >> View >> View ALL attached Drives. Look for the Upper Most Name of the Drive - Toshiba, probably.

Choose the Name of the Drive and Reformat as exFat with the Master Boot Records ( MBR for short ) and format it with the desired Name.

The original issue may have been a compromised or a corrupted Partition Map.

By doing the above operation - a New Partition Map has been put to the Drive and hopefully making the drive usable on Both Apple and Windows systems.

In what file format are the external drives formatted in ??

Usually, if they can not be mounted it is because the drives are formatted into a filing system the Apple Computer does not understand - like formatted like ext4 used in Linux or some other file format.

Apple can read NTFS from Windows but can not write to this without special Software installed on the Apple Computer.

User wrote " I have tested them on other computers (which are not Apple products)"

Can Disk Utilities even see the drives ?? If Yes, what file format are they ?? There might answer the question why it is not working.