Normally is a compatiblity thing of the HDD and PS4, i have the same problem with one HDD i use other NEW with 2 TB in exfat and never happend again (use Xvortex exploit to install PKG, or IDC with the correct payloads)
I'm using an older WD drive. I had to reformat my drive every time I wanted to install a new game. I also was only able to have one game on the drive at a time.
No idea why this is an issue. Hard drives and the PS4 just seem to be very finicky.
was happening to me but i shut down ps4 (off) unplugged it plugged it back in loaded exploit and working again was not my usb stick check your pgk file