Reset the +Volume Scheme+ as desired, so that you have the number of partitions you want, or select *1 partition* it you just want it to be one big volume. Be sure to select the DRIVE, and not the volume indented under the drive. Select the external drive in the sidebar. Run Disk Utility and connect the external drive. Note: This procedure will erase the data on the drive. If that WD external drive is one that is sold to Windows users, you may want to re-partition it anyways. Partition tab to partition a drive into two or more volumes. If you want to partition it so that one partition is only the Time Machine backup volume, that may be a good idea. You can use it to store other things, as long as it is used with the Mac to do so. So my advice is to partition and format it for Mac, and use it only with the Mac (if you are going to make it your Time Machine backup drive).
A drive that you use with Time Machine should be partition and formatted for Mac. If it is formatted for Windows and you use FAT32 as the format, you can use it with a Mac, but you should not use it as your Time Machine backup drive. To use with a Windows PC, it has to be formatted for Windows. However, if the 'other things' include using the drive with a Windows laptop, that may be a problem. You don't have to partition it to use it for Time Machine and other things.
