Bob meant Windows Explorer, not Internet Explorer, which is the Browser. Windows Explorer is the software you use to look at directories/folders and files in Windows.
With the Pono connected to your new laptop via a USB cable, the internal storage and SD Card on your Pono will look a bit like the storage in my Android phone, where "Disk" is the SD Card, and "Internal Shared Storage" is the memory hardware installed in the Pono, which in your case is called the "Pono Internal Memory Drive" by the sound of it. See the first attached image for what my Android device looks like.
So with the SD Card removed, select the "Pono Internal Memory Drive", select all the data in it, and press Shift+Delete to delete all the files and folders.
If you want to know how much space your high def, Flac and AIFF files will take up, and hence whether they will fit on your existing or new SD Card, open the "Audio > Files" View in MC, as per the second attached image, then click on the "flac" file type, then Ctrl+Click on the AIFF file type. Then at the bottom of MC in the center you will see how many files are selected, how long it would take to play them all, and how much space they take up. If all your files take up less than 64GB, then they would all fit on your old SD Card. If you can get that working again by formatting it then you may not need a new SD Card. Of course, a new SD Card may be faster.
MC wouldn't have broken the two 64GB cards you have, so they should still work. But maybe the way you worked out to transfer the high def files to the cards was the problem. You can just copy files to the SD Card using Windows Explorer with Copy/Paste or Drag & Drop, or you can use the Handheld Sync functionality in MC. The latter is more complicated, but it would allow you to transfer Playlists to the Pono as well if it supports them.
How did you try to "import music from my Cd library on JRiver's media player to my Pono"? Describe the process you used.
Bob assumed that you had an SD Card Reader that you can use with your SD Cards and the laptop. That is by far faster than transferring using a USB connection to the Pono. Do you have an SD Card Reader?