705 supports 64GB SDXC card on (slow) internal laptop reader, NOT on ext. USB30

Have just installed Knoppix 705 LiveCd to USBflash hoping to get support for a 64GB 400x SDXC card. 704 supported 16GB SDXC, but not the larger card, on a Delkin USB30 reader attached to a pci-expressbus adapter.

705 had the same problem. The card would appear fleetingly under three different names, N800, 64GB, or sdc1. But as soon as the cursor touched the link, it would vanish, sometimes generating a brief error message. Windows7 has no problem reading this card on the USB30 bus, but it has help from a proprietary driver for the Sabrent adapter.

The strange thing is that when I tried the card in the three year old Toshiba laptop's internal SD/XC slot, Knoppix 705 was able to read it there, albeit at a very slow 10MB/s.

Transfers through the USB30 port are closer to 50MB/s for files of this type (45MB .NEF).

I can't make any sense of the situation. The external reader is SDXC and UDMA compatible, while the internal reader's hardware probable predates those standards. And the internal reader's ability to read the file rules out the Sabrent driver making the difference.

Anyone care to venture a guess as to what's going on?