Try putting sellotape over the write-protect tab on the memory card.
No, really. This has fixed my three memory cards (three different manufacturers) all of which started to show the same problem -readable but not writeable - across three different card readers.
I always assumed that the write protect tab on an SD card was wired to the memory chip inside the card and prevented writing that way. But, I was wrong! Actually it's a lot more like VHS or floppy disk write protect - the write protect tab pushes up against a switch in the memory card slot, and it's the memory card reader that then prevents writing to the card.
If the switch in the card slot is dodgy, or not *perfectly* aligned with the tab on the memory card (for whatever reason) then you can't write to the card. Putting sellotape across the entire tab (wrapped around from one side of the card to the other, thus covering the tab) effectively gives the memory card switch something to "bite" on. Make sure there are no rough edges to the sellotape or it could get stuck in your card slot
RascalRuss, if you try this can you please post back and let us know if it works or not? I'm interested to see if it's the same problem because it's affected all my memory cards and readers - no combination works more than 25% of the time, if the sellotape is absent, so I'm interested to know if other people are experiencing the same problem. If this is a common problem then it'd be nice to publicise the solution because, I kid you not, it took me about 5 hours of diagnostics to come up with this fix. At one point I thought it was down to the Allocation size on the FAT table! *wrrrrong!*