[AprilHare]

I have a PRS-500 that has roughly 2100 books on a 8 GB memory stick, taking up about 1 GB of space.
Although the device is functioning, my problem is that when I go to add more books to my memory stick the Sony Connect software refuses to add more to the memory stick. It complains that the device is no longer attached when I try this. The internal memory still accepts files.
I have taken the memory stick and put it in a card reader connected to my Mac and it reads fine, although this doesn't allow me to add files.
I tried the memory stick on my Toshiba Satellite M100 and the memory stick icon appears in My Computer, however when I try to access the memory stick it asks me to insert a memory stick (?).
Any ideas?