When I connect an external drive to my PS4, it says "This USB storage device cannot be used"
It is likely the drive simply needs to be reformatted. Here are the steps to perform this operation on a Windows computer:
STEP 1:
a. Click on the Start Menu and type “diskpart” in the textbox, and press Enter. A DOS window will open.
b. Type “list disk” and press Enter.
c. Choose the drive by entering “select disk x”; for example “select disk 1” if it is listed as disk 1. Press Enter.
d. Type “clean” and press Enter..
e. The drive’s formatting will be removed. It will say “Successful” upon completion. Close the DOS window.
STEP 2:
a. Press the “Win + R” key combination to open RUN dialog box. Type "diskmgmt.msc", and press Enter. The Disk Management window will open. (You may receive an Initialize Disk prompt. If so, choose MBR)
b. In the lower-right section of the window, locate the disk from the list. Right-click on the Unallocated box and select New Simple Volume.
c. Click the the “Next” button until you arrive at the “Format Partition” step. Then, set the file system to “exFAT” and click “Next”
d. Click Finish. When the process is completed, the drive is identified with a drive letter and its status as Healthy. You can now use the new drive.