I had some trouble installing 4.2 on old hardware - it seems the underlying Debian Trixie has dropped support for several components, I could not get the install to complete on a Dell R510 with H700 perc card. I fully understand this is way beyond ideal hardware but what I thought was a full HBA card in a failed newer system turned out to be one of those mini daughter cards (thanks Dell! ), so I have to make do with the H700 in an older chassis as a stop-gap solution. I'm using 2 x fast SSD in hardware raid1 for boot and then the remaining 6 bays each as a RAID0 volume of an enterprise quality SSD which is presented to PBS for ZFS (RAIDZ2) for the data pool. Like I say - not ideal. But I'm also putting in a NAS for 'colder' archive backups in a remote building.
I was getting squashfs errors and it looked like the boot media was faulty. I also suspected it was using EFI rather than Legacy on the older chassis but that wasn't the issue either. I tried a lot of different permutations but came to the conclusion it wasn't possible to install 4.2. What worked was install older PBS 3.2 and upgrade to 4.2 - that was very smooth and no problems. Resulting system seems to be working better than I expected of such old hardware. Smartctl can see the smart data using megaraid so it's reasonably robust.
Anyway, just thought I'd share that 3.2 and upgrading is very smooth if you are using older hardware in a home lab etc. Its my first time trying PBS and I have to say I'm already a huge fan.