switched to mac, now Sony MS having problems, should I reformat it?

My Sony DSC-S75 was working fine with my PC. I got about 86 pics from my 128MB. I then switched to an iMac using iPhoto. The first time I plugged in my camera, I downloaded my photos with no problem, but when I went to use the camera again it said there was no room on the disk (despite also having no pictures on the camera). I then put the pictures back on the disk, plugged the camera back into my PC and did my normal download. This time my camera worked fine. The next time I tried to use my camera with my Mac, the pictures cleared off the disk again, but this time the disk space on the camera would only allow 50 pictures. Mac support people have no idea other than to reformat my memory stick. Any suggestions? And how do I reformat it if this is the right solution? Thanks.

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