ps2 and mem card problems

i recently bought my ps2 about 3 weeks ago. i picked up a used copy of castlevania: SotN yesterday. i tried to save on my sony 8mb mem card but it said memory card not found. i was like wtf. so i borrowed my friends 4mb one from ps1 and that also didnt work. the next day(today) i went over to another friends house and played castlevania on his ps1 and it saved perfectly fine on a 4mb mem card. i tried to save again on that same 4mb mem card again and it didnt work. what the hell? i thought everything was compatible.

I cant explain the 4mb card not working.
But you cant save PS1 games onto your PS2 card.
Only if you copy them accross in your browser screen. But then, thats just for backing up.

i had a worse prob than that, i got a memory card with my ps2 after about 3 weeks it loaded up fine i went to save and it said, please insert card. my card was completely wiped and couldnt save to it, i took it back and had to start all over again

Only SONY memory cards are compatible. Others (3rd parties) MAY work...or may not.

what if i use a sony ps1 mem card? will that work? if it doesnt, then ill just borrow a friends ps1 and play it through that. thanks for info guys