Yamaha charge a stupid amount for a replacement hard disk for a T4.
Although it is a recognised brand, there is nothing special about the disk itself and a replacement (250 GB 2.5 inch SATA) or larger one can be bought for less than $50.
However the disk requires a SATA to USB interface board to work with the Tyros. Yamaha will only sell you the disk complete with the interface board, which increases the cost (but not to Yamaha's extortionate price)!
If the interface board is still working on your existing T4 hard drive, then you can simply buy a new disk and transfer the board to it.
If the interface board has failed, then you need to either buy a replacement disk/interface from Yamaha, or obtain and use a different interface board.
The interface board on my T4 failed years ago, but the disk drive itself was still OK.
Rather than pay the silly price that Yamaha wanted for a replacement, I bought an external case for a USB laptop hard disk, deliberately looking for one with the same USB socket as the Yamaha interface, and one that I could easily remove the actual interface from.
I found such a device on Amazon for less than £10 GBP. I could have installed the original disk drive in the external case itself, but I wanted to actually mount the drive on the disk tray/caddy of the T4. Because I was able to remove the interface from the external case, this was simple to do.
The 'new' interface has worked fine since. The one that I bought was USB 2.0, but I have also tried a USB 3.0 one with no problems.
I have also since replaced the disk drive with an SSD with no problems although since the USB controller on the T4 is only USB 2.0 (and at times a very slow one at that!), this will restrict the apparent speed of the disk to USB 2, but in reality this is plenty fast enough for keyboard purposes. However I like to back up my T4 hard drive by removing it from the keyboard (just slide out the caddy) and connecting it directly to my PC via USB. The faster speed of the SSD and/or the USB 3 interface makes backing up the drive mush faster.
An SSD is also silent but the noise of a mechanical drive is not usually a problem in a keyboard! An SSD should be more immune to damage from physical shocks when it is working, although they are supposed to be susceptible to damage if the power is suddenly removed! You shouldn't need to defragment an SSD (in fact you shouldn't do this, since it increases wear on the cells making up the drive. SSDs also handle file deletion differently to mechanical drives.
Here is a link to a YouTube video showing how to change the disk on a T4 (also applies to T5). Although the video shows haw to replace the disk with an SSD, the steps are exactly the same if you just want to replace with a mechanical drive.
https://www.youtube.com/watch?v=ffeOJUp--1MImportant: the original interface on the Yamaha disk seems to be a bit sensitive to static shock, so if you remove the drive from the keyboard, make sure that the power is off, then leave it for a couple of minutes before removing it. It is best to minimise touching the interface board but you will have to if you want to change the drive, so ground yourself before touching the interface board.
A new drive must be formatted as FAT32 to work with the keyboard, and this can be done on the keyboard itself once the drive is fitted. This will also set up the folder that is used for holding 'multi' audio recordings. You don't need to concern yourself with labelling the drive!
Incidentally the T4 will work fine without a hard disk. The OS and settings files are all held in internal flash memory. However you will need one if you have any Yamaha Premium Voice Expansion packs. Non Yamaha expansion voices work fine.
Regards
Ian