MtothaJ is correct.... have a read of this HP advisory Document ID: c03993436 HERE.
The BIOS for the Z620 and Z420 is the same, and the ftp access to that HP SoftPaq for that early 1.23 BIOS install is below:
ftp://ftp.hp.com/pub/softpaq/sp64501-65000/sp64701.exe
If you have a Z820 there is a different link to fix the way-old BIOS issue for it too.
I personally like to update BIOS from within BIOS, as posted about here in the forum. Briefly, you run the SoftPaq, cancel out of it after it shows you all the ways to update, then go into the root level of C where you'll find the HP folder installed by the SoftPaq inside SWSetup folder, go into the DOS folder there and copy out the J61_0123.BIN file on to a thumb drive. Then boot into BIOS and choose to update the BIOS from the first tab entry available there. I forget the term used in BIOS, but it is obvious there. This is safer than updating BIOS from within your OS, which seems to work fine too.
Once you have that done you can get the latest, which 1/17 currently is 3.91 BIOS HERE and run that SP78208 the same way.
If you are already above 1.23 then use my method of updating BIOS from within BIOS. If there is some program in your OS that is preventing that you can harvest the .BIN file on another box to a thumb drive there and bring that on over for use during boot of your Z620. Updating BIOS from within BIOS is using a very primative operating system (called BIOS) and that is done before anything in your regular OS can interfere with the process.
That should do it......