1) BENCH mode. I'm not an expert regarding bench mode or the hidden diagnostic mode and I was unable to find any documentation, but I can give you this information:
BENCH mode (installing the bench jumper on the Procurve 5400 zl Switch Management Module (J8726A)) appears to put the system into a bench test mode for testing the hardware.
in version K.16.02.0014 there are 67 possible commands shown in help in "normal mode". In edomtset mode there are 849 commands shown in help and in bench mode there are 381 commands listed in help. Bench adds the commands shown in the next message sorted in alpha order (sorry 25,000 character limit on posts and you can't attach a text file.)
This mode, like hidden diagnostic mode, has plenty of dangerous commands.... Be careful. This mode helped in my troubleshooting only to allow me to mount a CompactFlash disk during boot when the management module contained faulty memory. So, my guess is that this mode skips some file integrity tests.
2) Is there any other way that is worth knowing about? :)
3) I actually ran the following commands to ensure the flash was good and then format it in whole disk mode
badblocks -svw /dev/sdX
mkfs.fat -F 32 -I /dev/sdX
Thanks for pointing out the '-c' flag to mkfs.fat
4) The RAM in all of the managment modules I have are are at least PC2700 and one is PC3200. The bad stick was a 512MB (the rest are 256MB) and it had a Dell sticker on it though it is a Micron part. I wish I still had a machine that I could test DDR RAM with so that I could verify, but alas those have long gone to Goodwill. AFAIKT all of the DIMMs that I have are not "HP" and that does not seem to make a difference. I just had a bad stick of RAM, it happens. A stick of Micron PC2700 should be fine, it's not like the processor in the Management Module is running that fast.