Ok, i hope this helps you out a bit. The first thing you'll need to be aware of is that this process will erase everything on your MyBook, so be sure to transfer everything you wish to keep onto another hard drive (or computer) temporarily.
Everything you need for this whole process is in an app included in OSX called "Disk Utility". You can probably just type that into spotlight, or you can open finder and go to applications->utilities->Disk Utility.
The first thing i would do there is exactly what i mentioned earlier. Erase everything and start from scratch. To do this Click on the drive in the left column (mine says "37.3 GB Samsung..."), click on the "Erase" tab at the top, and pick "Mac OS Extended (Journaled)". You can name it if you want, but it'll get overwritten later anyway. Then click the "erase" button.
Once that finishes, click on the drive again (the top one where mine says "37.3 GB Samsung...", not the one under it). Then click on the "Partition" tab, and under "Volume Scheme" choose "2 Partitions" (although you can certainly make more if you wanted to). Slide the middle bar around to adjust the relative size of the two partitions. Then click "Apply", and "Partition" in the windows that pops up.
Next we need to rename the partitions, and change one of them to FAT32. To do this, in the left column, click on the partition you want to use as your Time Machine Backup (where mine says "Untitled 1"). We don't have to reformat this one, so we'll just rename it something a little more helpful for identification. Go to the "Erase" tab, enter a new name, and then click the "Erase" button.
Next, we'll reformat and rename the Partition you want to use to transfer files. So select the other partition in the left column (where mine says "Untitled 2"), and then click on the "Erase" tab. For this partition, we will have to choose a different "Volume Format". Choose "MS-DOS (FAT)" here. Then enter a name for the partition, and finally click on the "Erase" button.
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