on time machine backup it stops and says disk is full when there is available space

Hello frietch2,

Welcome to Apple Support Communities!

If we understand your post correctly, Time Machine stops when attempting to backup to an external drive with a message that says the disk is full when you know there is space available. We're happy to help!

Did you check the article that you came from to ensure that the external hard drive you're using is compatible?

Backup disks you can use with Time Machine

The article below will help when you have an issue with backing up using Time Machine.

If you can't back up or restore your Mac using Time Machine 

Here is another article for help troubleshooting Time Machine:

Time Machine troubleshooting on Mac

Let us know if you have any questions.

Thank you for using Apple Support Communities.

Take care!

Check if your internal hard drive is nearly full. This can cause an error