Question on new HP rack mount servers

I don't think so... about the statement "drives must be HP branded hard drives " always the servers some with compatible Harddisks only... it may be HP or any other manufacturers.

we are using since a very long time without any issue... on non HP HDD's

My rep stated that because HP used a proprietary driver, and the drives came with the mounting cases, that only HP drives would work.  So, if I could get the mounting cases, I could use my own drives?  I have older HP Proliant rack mount servers that I can install any drive into, so that is why I thought I could use any drive.  However, those servers are ~20 years old at this point.

Some companies play these sorts of tricks.

Simple test, buy a bare drive + plug it in + see if it works.

18TB EXOS drives are <$400 USD on Amazon, likely a simple test is easy way to determine if you must buy HP drives.

Since you say, "We are looking at purchasing..."

Tip: Avoid vendors like HP that play these games. Buy a generic rack device, which has none of the HP-esque nonsense... as generic means you can add any hardware, including disks, however you like.

AFAIK the problem is not the generic disks but the smart drive carriers, HPE won't sell them separately and the LEDs don't work if not HPE certified carrier so 3rd party ones don't work.

You mentioned, "AFAIK the problem is not the generic disks but the smart drive carriers, HPE won't sell them separately and the LEDs don't work if not HPE certified carrier so 3rd party ones don't work."

Ugh... proprietary nonsense...

I've now added HP to my "no fly list", as this type of nonsense is just unacceptable...

A long term solution using generic tech seems like the most flexible approach...

I may be wrong, but I read it somewhere - will look for the thread.

Hard drives direct have the carriers, seem to be genuine HPE. https://www.harddrivesdirect.com/product_info.php?products_id=200660_651687-001¤cy=USD

Here it is, maybe they backed out and newer firmware enables the LEDs again with 3rd party carriers but it's a risk you take.
https://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c05382373

The firmware on the HP branded disks is set to work best with the SmartArray RAID controllers HP servers use. Other disks can work, but not always. But most important is to use Disks that are built for RAID use. You can't just use any consumer brand disks in a RAID array & expect them to work. Also, don't get Disks that are meant for NAS use.

The disk caddies can be bought empty & separately, but you will need them, & you must also make sure they are the right caddy's for your Server Model. There can be different versions for other HP server Models. Also make sure you get the correct Form-Factor of Disks (3.5" or 2.5").