What did you format the PC drive as? How old is your Mac? Modern Macs with OS10.6.8 (Snow Leopard) and above should be able to read FAT16-, FAT32- and NTFS-format drives. I remember an Intel Quad being unable to read an NTFS drive about two years ago, but that seems to be in the past.
Next time, test it before you put the P2 files onto the drive. When a drive is mis-formatted, the only solution is usually to re-format the drive, and that wipes all the files on it.
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