You state the built-in Windows Disk Optimizer won't let you defrag the drive, that's just the GUI interface that stops you from defragmenting certain file systems or drive types (such as it won't let you defrag a USB Flash Drive), whereas via an admin Command Prompt you can use the command line defrag regardless of disk type or file system that's supported by Windows, I use it all the time.
I know I'm extraordinarily late, but this is incorrect. The windows disk optimizer supports NTFS and FAT32, but it does not support exFAT in any capacity whatsoever. OP would need (and hopefully found) a defragger that supports exFAT, such as O&O.
Defragment takes too long on exFAT external drive
in Defraggler
Posted
I know I'm extraordinarily late, but this is incorrect. The windows disk optimizer supports NTFS and FAT32, but it does not support exFAT in any capacity whatsoever. OP would need (and hopefully found) a defragger that supports exFAT, such as O&O.