Some portable drives can be disassembled and the drive inside removed and used.
Not the answer you wanted, but the only way I know the NVR will recognize it.
USB is far slower than SATA that the NVR wants to use.
Second part is that the NVR (at least my Dahua NVR) does not allow recording directly to the USB drive.
The USB drive option is only for backing up selected footage.
You've spent loads on a 4k system, you must have the money for a surveillance rated HDD.
Save yourself the headache and do it right. As mentioned, USB does not offer the necessary throughput. Also, the drive that’s inside your portable housing is not going to be surveillance rated. Get yourself some Western Digital Purple drives. If you think about it, the hard drive is the most important part of the system. The rest of the equipment is useless if you don’t have the footage recorded. Unless your system is meant for live monitoring...