Why cant i drag and drop files from my mac to my seagate hard drive?

I have a Seagate FreeAgent GoFlex external portable hard drive. I had no expectations to use it as a back up, just a storage device, so i erased all the software on the hard drive before i started using it.

It is a portable external hard drive that i use with three computers that i constantly work with every day. A Windows 7 comp, and two different macs. I plugged it into my Windows computer first, and thats were i erased the software. I then uploaded my files from my computer to my hard drive, with no problems. I then took it to school and plugged it into my mac at school, and copied the files from my hard drive to my mac at school with out any issues. it worked fine, and i even saved a few files on my hard drive. When i unplugged it and took it to my computer at work, which is a mac as well, i can now only READ the files from my hard drive. I can no longer save files to my hard drive, nor can i take files from my work computer and drag and drop them into my hard drive.

I opened a file off my hard drive in photoshop on my work mac, and worked on it. I tried to save it but, it said that i had no access to save onto my hard drive. I then opened up an html file in dreamweaver and it said that "write access was denied" on my hard drive. it also happens in final cut pro, flash and illustrator. It wont even upload files to my website using the FileZilla Agent.

I'm very confused as to why my hard drive will not work with my work Mac but works fine on my schools Mac and my home computer which is a windows. Does anyone know what might be wrong? I doubt there is something wrong with my work computer, it's always worked fine before and with other hard drives. My hard drive works on two out of the three computers i use, so i seriously have no idea what might be wrong.