We need to replace an appliance's drive that has failed and we don't have IBM's support in this particular case (EOS).

My Question is: Does the appliance will take a similar drive if it is not the same Part Number that the one the current disk (the one failed) have? Or we 'must' use a replacement drive with the same part number? I need clarification on this to see if we can go ahead. I know for experience that there are devices that are really picky when it comes this kind of approach so I need clarification.

The appliance takes a drive with Part Number 81Y9727 listed here: https://www.ibm.com/support/pages/hard-drive-accessories-ibm-system-x3530-m4

I was redirected by support to a Lenovo datacenter website, and, while I was able to find the drive, it's no longer being sold (the unit appeared on screen) but no option to purchase it is available.

We just need to know if we can go ahead with a different drive's Part Number to evaluate our options from that point on.

Typically, with most hardware if there is not an exact part number available, then you can use an equivalent. The specs must match though. We have seen Lenovo RAIDs using different part numbers and working. It's important that they are of similar spec.

Recommendation: It is always recommended to combine into a single RAID drive group the drives of the same type, rotational speed, and size. For info about Lenovo RAID configuration and speeds, see: https://lenovopress.com/lp0578.pdf

The QRadar Hardware Guide has specs in it too: https://www.ibm.com/docs/en/qsip/7.4?topic=hardware-qradar-m4-appliance-overview