One further reason to reformat is if you may use the drive for Time Machine.
As of September 2018 there's no direct official support for exFAT according to this Apple support document
Unofficially, you can create a sparsebundle as (described on MacOSHints](http://hints.macworld.com/article.php?story=20140415132734925)
First, connect the unsupported volume (in this case, an exFAT external hard drive.) When it mounts, open the Terminal and type these commands, substituting 'My External HDD Name' for the name of the unsupported volume.
cd 'My External HDD Name'
Next, type this code, substituting for your needs:
hdiutil create -size 320g -type SPARSEBUNDLE -fs "HFS+J" MacBook-Backup.sparsebundle
Here, a 320GB sparse bundle named 'MacBook-Backup' is being made and mounted. You can change these values as you see fit. From herein, I'll refer to the sparse bundle name as 'MacBook-Backup'.
After you've run these commands, a new volume named untitled will appear on your Desktop. This will become your Time Machine backup volume. If you want, rename it to something else (I called mine MacBook Pro Backup) and run the command:
You should see a list appear of all connected volumes. Find your new volume's name and read along until you find the disk identifier. In this case, my identifier is disk2s2, but yours may be different.
Finally, enter the commands below (entering your password if prompted). Replace disk2s2 with your identifier, and 'MacBook Pro Backup' with the name of your new Time Machine volume.
sudo diskutil enableOwnership /dev/disk2s2
sudo tmutil setdestination '/Volumes/MacBook Pro Backup'