Hello there, squash1.
The following Knowledge Base article offers up some great information for the questions you are asking:
In regards to stopping the backup:
You may want to set up Time Machine in the evening so that the initial backup can be done overnight. It may take a while, depending on the size of your OS X volume. You should not interrupt the initial backup. You can continue to use your Mac while Time Machine backs up.
Once the initial backup is completed, Time Machine performs subsequent hourly backups of only the files that have changed on your Mac since the last backup (as long as your backup drive is connected).
Tip: You can manually initiate a Time Machine backup cycle at any time by choosing Back up Now from the Time Machine menu, even if you have Time Machine preferences set to off.
There are some good recommendations as well here, although they are written for the Time Capsule :
About the first backup to an AirPort Time CapsuleYour initial backup may be faster if you leave your computer in the same room as the AirPort Time Capsule, or use an Ethernet cable to connect your Mac to one of the Ethernet ports on the AirPort Time Capsule. You should not interrupt the initial backup connection by sleeping or shutting down the computer. You can continue to use your Mac while Time Machine backs up. Additional information is available about backing up with an AirPort Time Capsule for the first time.
Once the initial backup is completed, Time Machine performs subsequent backups of only the files that have changed on your Mac since the last backup was performed. Subsequent backups happen when a connection between the Mac and the backup drive is available. You can manually initiate a Time Machine backup cycle by selecting "Back up Now" from the Time Machine menu in the menu bar.
Thanks for reaching out to Apple Support Communities.Cheers,
Thank you Pedro for your reply, this helps.
Unfortunately. I had to interrupt the first back up as it was taken over week and I needed to take it out.
During the first back up. it had backed up about 70GB out of 260GB, 260GB being the original disc size. On the subsequent back up, it started backing up again, but this time, it was only trying to back up 190GB, i,e, the remaining data.
What are the implications of this, you mentioned in your original respond, you should not interupt you first back up, will I be able to do a full restore? Even if I restore the total disc over multiple back ups, can I restore any file?