Time Machine won't back up

I Ihave a 1 TB Lacie hard drive for Timemachine/
It worked perfectly for months then I started getting error messages

You can still open or copy files on the disk, but you can’t save changes to files on the disk. Back up the disk and reformat it as soon as you can.

and

Files can’t be copied onto the backup disk because it appears to be read-only.

You may need to repair or reformat the disk using Disk Utility. If the disk can’t be repaired, you must use a different disk for backups. Open Time Machine preferences to select a different backup disk.

I have replaced my mac book pro hard drive ...
I have reformatted the ext hard drive MANY times. It always works perfectly for a few days to a week then I get the error again
Its driving me crazy.

Any advice greatly appreciated!

Thanks

 

Thanks so much for that info. Lacie are replacing it which is great.