iMac keeps ejecting memory card before files are transferred

I have an iMac Late 2012 running Yosemite 10.10.5

When I try to transfer files from an SD card or CF memory card, the card ejects part way through and the files don't transfer. I try several times and still no luck. I thought it was my card reader so I bought another one. Thought it was my CF but does the same thing with the SD card. I even have the same issue on my MacBook.

I found some older posts on this forum complaining about the same issue but haven't found any fixes.

Please help!!! It's too late for me to buy Applecare 😟

Thanks