Why won't external HD allow writing on separate macs?

[knobtwiddler]

I just bought a new Seagate External HD to free up some space on my mac and macbook. I installed it on my mac and then tried to move files from the macbook onto it but get a message saying I can't modify it. What am I supposed to do. SHould I install the software that came with it onto my macbook. Hopefully, I've answered my own question here. If I can't use it on multiple computers than what good is it?

 

[jim.lynch]

Possibly it's pre-formatted as NTFS. Your Mac won't be able to write to this without additional software (MacFuse NTFS-3g).
Try reformatting the drive with Disk Utility, format as HFS+ if you will only ever use it with a Mac, or as MS-DOS (FAT32) if you want compatibility with all computers.
I would avoid the existing software packaged with the disk, it's usually unnecessary.

 

[jackerin]

If it works on one Mac and not another it sounds like a file permission problem. In the Finder, get info on the drive and see who the "owner" user is and who has write privilegies. If you see a checkbox to ignore all file permissions, click it and see if that resolves it.