Can I install Power Path software if the devices already have data?

Installing Powerpath will not cause data loss. Powerpath does not touch the data already on the drives

Yes, The server has two HBAs.

PowerPath seat below windows driver ..so should be no problem. Do your system have dual HBAs ? I am suprised you have not experience corruption if they are connected to two path on the storage side.

Thank you so much.

Windows 2000 does not handle multiple paths well without PowerPath, so if the host has been running multiple paths without PowerPath you need to run a chdsk on each of those volumes to check for filesystem issues. Do a backup beforehand, in case chkdsk does find issues.

I am wondering about the 2 HBA's which are in use up until now, since Windows usually does a great job corrupting data is there's no PPath available. Windows sees 2 disks, which are in fact the same lun, but it tries to put a signature on both... I've seen problems with this in the past.

On the other hand: I've also seen hosts with this issue (no PPath installed) where somehow a lun is only seen just once. Never found out why Windows only say the lun once instead of a few times, one for each HBA.

indeed, you might need to restart the server once after deploying the PowerPath

You DO need a reboot. PPath asks for it, so you can't miss it.