There are no official 64MB Memory Cards. The only official Memory Card produced by Sony was only 8MB in capacity.
I bought a 64 MB Sony memory card.
It's quite likely that the card you have bought is actually an imitation, but with a capacity of 64MB.
Try formatting it... and it may solve your problems if the problems you have was caused by filesystem corruption on the card. I think that all the memory cards (2 Original SCE 8MB PS2 MCs, and a DragonPlus PS1 MC) come unformatted, and you were required to format them before use (The PS2 browser asks me to format them).
I bougt another two, and i have a problem. Cant install anything, unable to copy even a savegames from mine to them. Only says Copy failed.
I am not sure what you can use to format your card, but I know that the FMCB installer can format Memory Cards.
Or it could have just been that the card is a defect (Or you have been cheated).
That "unknown" one is Hynix. They seem to manufacture memory chips too.
Two of them are SAMSUNG, one is ... i cant even read ....
SCPH-10000 S. MINOKAMO v1.01 (Defunct)
SCPH-10000 S. KISARAZU v1.00 (faulty)
SCPH-15000 S. KOHDA (With warranty seal!) :D
DESR-5100 S. EMCS