The documentation for the X400 is here: https://support.emc.com/docu44413
As far as finding a replacement part, you have a couple options. You can do a Time and Materials quote for the part replacement through Isilon to have them replace it. They will need the serial number of the node in question.
If you are looking to purchase a boot drive from a third party, I don’t know that the Isilon part number would be valid outside of Isilon’s dispatch system.You could however find a compatible model number. If you know what firmware version you are on, check the release notes. Otherwise you should be able to see what model you currently have with this command:
#isi_radish -q /dev/ad[0,1,2,3,4,7]
As a side note, if you are running anything below 8.0, you can use [2,3,4,7]. In 8.0 OneFS started streamlining the boot drive names to ad0 and ad1. The command above should work on your X400 though. It doesn’t change anything, It just provides an informational output.
Just know that if you do go the third party route, Isilon won’t assume any liability for it.