I cannot import Raw images from 32 GB SDHC from card reader. I have Aperture 3.6 and El Capitan.

After hours of trying I find that I cannot import Raw images into my iMac from either of two card readers. My current memory chip is a 32 GB Sony 95 MB/s. It has worked with much effort in the past. Effort being: reboot the iMac, reboot Aperture, try Import after no automatic import. Now Photos automatically comes up and I quit Photos and try to proceed with Aperture. It shows Loading Assets but nothing happens. The SDHC has room for about 1200 20MB photos with about 400 left still to be taken. My iMac is a 24-inch with El Capitan. I bought it in 2009.