Hey there, oldwoody1.
Basically many phones and tablets won't recognize a memory card, which has been formatted via a computer. It's OK for the SD card not to be detected as a storage location where you can write files on, but is it detected by the tablet at all? Go to settings -> storage and scroll down a bit to see if you have the option to mount or format (note that formatting the memory card will delete all the data on it) the SD card. If you you have such an option, go ahead and do it, to see if the card pops-up as a storage location. Since I don't know the version of Android your tablet is using and since most manufacturers modify it a bit, the option I've mentioned might not be at the exact specific location I've mentioned. This...