moving JPEGS to external HD

Recently new to mac and liking most things so far but, today I was downloading a RAW file into aperture and everything else (iphoto, preview) then wanted to move it (master) to my external HD. I got this message while trying to--

Picture could not be saved because write access was not granted.

The folders and files in the external HD were created with my old PC (XP).

When I import the files from the external HD they open up in iphoto & aperture no problem but going the other way it doesn't work for some reason.

Any ideas?

The hard drive is formatted in NTFS (the default format for Windows XP/Vista) and OS X only has read access to NTFS volumes. Either reformat in MS-DOS (FAT32) or use a program like Paragon NTFS for Mac to give the Mac full read/write support.