|Apparently, the F5U248 first came out in 2003.|
Lots of older card readers can't recognize memory cards that have a capacity of over 2gb, sometimes over 1gb.
I have two of them that can't read cards > 2gb, one newer one that can.
"When I put a loaded, unlocked SDHC card in..."
The F5U248 supports reading SD cards, but does not support reading SD HC cards.
No mention of SD HC cards in it's manual:
SD 1gb and 2gb limits; SD HC limits