USB Buffalo500G is no longer compatible with my mbpr

[Appletise]

can anyone tell me why my Buffalo500G is no longer compatible? l made it compatible with mac by reformatting to FAT32 all fine for 4months when l wanted to use. l hardly used it, like only used it few times to back up about 4 months ago when l made it compatible with my mbpr. its strange it is not recognised in the left usb socket so l plugged it in the right USB socket, where it is recognised but a small window appears and says: "OS X can't repair the disk "Buffalo500G" you can still open or copy files on the disk, but can't save changes to files on the disk. Back up the disk and reformat it as soon as you can"
is this occurence typical for USB HDs made for pcs and reformatted to FAT32 for mac?

thanks