I too had a portable SD card problem when first trying to flash AEX. The card was detected in my case, but was shown as being corrupted (which it was not). So my fix may or may not help in your case. What work'd for me was to restore a previous TWRP backup image of Danil's j33 build of Pie, on which my portable card was working fine. Then I dirty flash'd AEX upon that, and the portable card was fully recognized.