What Is Actually Wrong With It

Before the decision

Is the drive the fault at all?

Ten seconds, and it can save you the whole rebuild. There is one fault a rebuild definitely fixes, and one where it changes nothing at all.

Did the player show READ ONLY? AlphaTheta documents that as the drive being write-protected, or its file system having failed. Their remedy is another USB device, or backing the data up to a computer and reformatting this one. That is the one case where their documentation asks you to reformat at all.
Will the drive take a new file? Make an empty folder on it from the computer. If that fails, the file system has failed.
Check what the drive actually is. Mac: Disk Utility, View, Show All Devices. Windows: Disk Management. Get Info and right-click Properties do not show the partition scheme, and the scheme is half the answer.
You want two things: the file system (FAT32, exFAT, NTFS, HFS+) and the scheme (MBR, or Mac GUID / GPT).

exFAT is not automatically the fault. The only decks that refuse it are the older ones, the CDJ-2000, CDJ-2000NXS and CDJ-2000NXS2, the CDJ-900, CDJ-900NXS and CDJ-850, the XDJ-700, the XDJ-1000MK2, the XDJ-RX, the XDJ-RX2 and the XDJ-RR, and there the rebuild is the fix. Every newer player reads exFAT, the CDJ-3000, XDJ-RX3 and XDJ-XZ from firmware 1.20, 1.11 and 1.23, so on those a rebuild changes nothing. NTFS and the Mac GUID / GPT scheme are the two that are never supported: AlphaTheta says so in the same words for the CDJ-3000X, the XDJ-RX3 and the XDJ-AZ.

What did you find?

You follow these steps at your own risk. Full disclaimer.