^Thanks so much for the advice JonoMarshall, shall give it a go!
Actually, as long as the hard drive you have is larger than the hard drive in the iMac, you don't even need to partition it.
Once time machine has finished backing up, it'll have a folder the size of the used part of your hard drive, but you'll be able to use the remaining space as you see fit.
The advantage of partitioning is that it may avoid complications with backing up once the hard drive is quite full.
Actually, as long as the hard drive you have is larger than the hard drive in the iMac, you don't even need to partition it.
Once time machine has finished backing up, it'll have a folder the size of the used part of your hard drive, but you'll be able to use the remaining space as you see fit.
The advantage of partitioning is that it may avoid complications with backing up once the hard drive is quite full.