a friend of mine who runs a computer shop, sent message that his external hard drive is not accessible on a brand new PC, while old PC is still able to run it, but data reading was slow. he sent me photo that new PC was showing  “Disk unknown not initialized” message when he checked in Disk Management. and if he try to initialize it asks in which format you want to initialize MBR or GPT.

Disk unknown not initialized

issue was not the initialization of disk, important thing was to save the data on this external HDD which was around 1 TB. Because initialization of hard disk will remove all data & it will be almost unrecoverable. 
In some cases external drives show such errors when we use poor quality cables to connect and it does not provide enough power to operate properly, some drives start working if we plug them in USB3 port instead of USB2 port.

But if all these suggestions fail, that means your partition table has been corrupted, there are several ways and software to recover your partition. but the one which is most easiest and free is using Mini Tool Partition Wizard. which can be download for free from https://www.partitionwizard.com/free-partition-manager.html

After installation connect your HDD with the PC and open Mini Tool Partition Wizard, then select the HDD which have problem, right click on it and chose partition recovery. then follow on screen wizard to complete recovery process.

Disk unknown not initialized

after recovery you will be able to access your data on this HDD, if you find your HDD is running slow, then you must copy your data on some other drive & format your external hard drive,
hopefully it will continue to work normally.