Most likely the problem has to do with the storage capacity.
I have no experience with Memory Sticks but other memory cards (SD, Compact Flash) use a different formatting when you get above 2 gig cards. (SD cards above 2 gig are called HC for High Capacity).
The Epson Photo Stylus Photo RX500 apparently can not read a High Capacity memory stick.
Looks like you need to buy a 2GB (or smaller) memory stick for your H50, if you want to print directly from the card.
Plus you made a mistake.
Anytime you take a card from one camera and plan to use it in a different camera, you should format the card IN the different camera first. That also goes for a new unused card.
To recover your 128MB memory stick, this usually works:
Use a computer with a card reader.
Insert memory stick and look at card reader/memory stick with Windows Explorer or My Computer
Delete all files and folders you find on the card.
Remove card and insert it into a camera.
You should now be able to format the card with the camera.
I just purchased Sony DSC H50 after being pleased with Sony DSC S70.
When I put the "Sony memory stick PRO Duo 4GB Magic Gate" fom H50 into my Epson Photo Stylus Photo RX500 it was not recognized. "Sony memory stick Duo Magic Gate 128MB" from S70 has been working.
Then I put the 128MB into the new H50 shot a couple of shots and Epson did not recognize (actually it says something like needs formating but won't accept the format command)
Put the 128MB back into S70 and now the camera does not recognize the stick. Can't even let the S70 format it.
Does the new H50 format the sticks with a "new & improved" format?
Seem like "MAGIC GATE" indicates "NOW YOU SEE IT NOW YOU DON'T"
Thanks for any and all assistance with this discusting problem.