Exteal hard drive not functioning properly on Mac but fine on PC

I have a WD My Passport Ultra (1TB) hard drive I bought a year ago, which originally has two partitions, one for Time Machine backups and the other for my documents. The Time Machine partition is formatted in OS X Extended (Joualed), and the documents partition in exFAT, for easier access on both my Mac and my PC.

After accidentally mistreating it a few times, the hard drive is starting to act up on my Mac. The Time Machine partition refuses to mount altogether, and r/w speed of the documents partition is excruciatingly slow. I tried First Aid and erasing both the partitions using my Mac but failed.

Strangely, Windows correctly registered both partitions, and after erasing and reformatting "Time Machine" to exFAT, neither seems to have issues with r/w speed.

Connecting the hard drive to my Mac once again (now that both of the partitions are in exFAT) and launching Disk Utility shows only the documents partition, whose r/w speed is still terrible. The "Time Machine" is greyed out and unable to be mounted. Erasing or "First Aid" hangs for a very long time and then fails as before.

I'm not so worried about my data, as I managed to create a backup on my Windows machine. Just really curious as to what caused this peculiar situation, and possible solutions.

I'm running Windows 7 on my PC, and OS X 10.11.4 on my rMBP. Happy to provide any missing information.

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