I have come up against this several times when using HP optical drives such as DVD-ROM and DVD-ReadWrite devices. In relatively rare cases the official W7Pro64 installer DVD did not have a driver that would allow the installed optical drive to work.
The solution for me in those cases was to swap in a nother known-good HP optical drive and load from that. I also have posted here about a certain HP DVD-ROM that HP did not have a firmware updater for, but Dell did, and the description from Dell included info that the update made the drive W7 compatible. I "cross-flashed" that firmware onto the HP drive, and it now shows up as a Dell OD. And, now it can load W7Pro-64 just fine.
I have seen this issue in several xw era workstations, and on one Z620. I'd try that first, and you can buy near new HP SATA optical drives off eBay inexpensively. A random buy of one will likely fix your issue, given how rarely I have seen this. Ideally get a higher HP part number one. IDE optical drives seem to never or almost never have this issue. Another option is to load from an external USB-connected optical drive, and you'll need to set BIOS so it can boot from USB in that case.
A bit off topic..... always load the OS with SATA emulation in BIOS set to "RAID + AHCI" (and a few HP workstations don't give that option but if you set it to "RAID" you get the drivers for both RAID and AHCI anyway, which is the intent). You really want the drivers for both to be loaded automatically through this setting, and that BIOS setting is what Intel and HP recommend, and what comes as the default from the HP factory.