THE DENON WILL NOT
READ THE DRIVE

• The file system. exFAT or FAT32, and nothing else. Denon mark exFAT "(recommended)" in the supported file systems line. NTFS is not on Denon's list anywhere, and a Windows machine hands you NTFS by default on a large drive. The full format answer is here.
• The other port, and the other slot. If the deck has two USB ports and an SD slot, try each one. A dead port reads exactly like a dead drive.
• Give it time. A large library is not instant. Count to thirty before you decide.
• Power cycle the deck, not just the drive.

If none of that changes anything, stop guessing about the drive. The next section is where the answer usually is.

This is the one the forums mention last and the manufacturer mentions first.

Denon state it as a rule rather than a suggestion. Drives get ejected, never pulled. The guide states it as a rule: "To prevent data corruption, drives must be ejected before being removed from the player." Ejecting a device unloads its tracks from every networked media player, and if a track from it is playing the unit asks before it stops that track.

On an SC deck the eject is a real step with a real screen. Press Media Eject, then tap the Eject icon beside the device name on the touchscreen. Denon ask first that you "make sure none of its tracks are being played on any connected PRIME hardware units", and if one is playing the deck asks whether to continue, which stops the track.

A DJ at the end of a set, in the dark, with the next DJ waiting, pulls the stick. It works ninety nine times. The hundredth time it happens during a write, and the drive that comes home is the drive that will not read next week. Nothing warns you at the moment it happens, which is why the cause and the symptom feel unrelated.

If the deck mounts the drive but shows no library, or shows one that is wrong or empty, this is the section. Engine DJ document both the causes and the recovery, and it is worth reading in their words rather than a forum's.

Engine DJ publish the causes of a corrupted Engine library and the recovery for a drive. The causes they name are force ejecting a drive, particularly during an import or a sync, using third party tools on the database file, switching the device to standalone mode before the drive has ejected successfully, and storing the drive near magnets. The recovery for an external drive is to close Engine DJ, go to the root of the drive, delete the Database2 folder inside the Engine Library folder, restart the software and resync from Sync Manager.

Read the causes again, because three of the four are things a DJ does routinely. Force ejecting during an import or a sync. Switching the unit to standalone before the drive has finished ejecting. And magnets, which is a bag full of headphones and a laptop sleeve with a magnetic closure.

The recovery deletes a folder, not your music. The Database2 folder holds the index. The audio files sit beside it and are untouched. You rebuild the index by resyncing from Sync Manager, which is why this is a twenty minute problem at home and an unsolvable one in a booth.

All three Engine OS brands publish the same recommendation, in almost the same words, and it is more specific than "use a good one".

Denon name the drives they want rather than leaving it to chance: "We recommend using only high-quality, name brand USB 3.0 or Class U1 or U3 SD cards as source drives, especially when cloud services are enabled."

Two things in that sentence do work. USB 3.0, because the sustained read a large library needs is not what a cheap USB 2.0 stick delivers. And especially when cloud services are enabled, which is the maker saying plainly that a marginal drive fails first under the heaviest load.

On an SC deck there is a second drive to think about. Denon specify nearly any standard 2.5 inch SATA drive, solid state or hard disk, and Denon require that it "uses (or can use) the exFAT or FAT32 file systems". A drive that never leaves the deck never gets pulled in the dark, which is the single best argument for using the internal bay.

Nothing above is a booth fix. Deleting a database folder and resyncing needs a computer and twenty minutes you do not have.

What you do have: the music files are almost certainly fine. A corrupted Engine library is a broken index sitting next to intact audio. If the deck will browse the drive as folders, play from the folders. You lose your playlists, cues and grids, and you do not lose the set.

Then fix it properly tomorrow, and eject the drive properly from now on.